Clerk is indeed clerical worker. With its commerce, Liverpool needed many such. TF is Territorial Force, probably the nearest thing to explain it to you would be it's like the National Guard. Civilian, who trained for a certain number of days a year as soldiers and could be called up in time of war. Later became the Territorial Army and is now called the Army Reserve.
During the war, infantry regiments comprised of four different types of battalion. Regular Battalions- usually the 1st and 2nd; TF battalions, usually the 4th and 5th (the KLR and London Regiment were two exceptions to these generalities, the KLR actually ending up with 50 battalions- Wyrall says 49, but I've counted 50); 'Kitchener'or Service battalions; and various depot/ training/ or reserve battalions. The 3rd Bn was usually the Reserve Battalion. Only the Regular, TF and Service Battalions served overseas in combat roles, though some of the Garrison Battalions were posted overseas too. (Garrison Battalions were usually made up of men whose age or fitness precluded combat, but who could still make a contribution.)
If you're confused by the numbering during WW1, men in regular infantry battalions had a 4-figure number. TF battalions began with '1' in 1908 and as more enlisted, worked their way up to a 4-figure number, e.g. 4785. Numbers were battalion numbers and if a man moved regiments, he was given a new number. Men who were conscripted, or volunteered for the Kitchener battalions were given a 5-figure number. By the end of 1916, it became clear that 4-figures just weren't enough for the TF and TF soldiers were given a new 6-figure number on 1st March 1917. Usually, but not always, the numbers indicated the battalion, for example, 200123 would be 4th Bn, 240123, 5th Bn. Men who were missing in action were not usually declared dead until a year had passed, unless other information came in to suggest otherwise. Few of the bodies of those killed at Guillemont in August 1916 were found and identified, and a 6-figure number for anyone killed then indicates that his fate was unknown by authorities. The fighting at Guillemont was bitter and even the graves of men buried some way behind the lines were obliterated by the shelling. In that little bit of cornfield in my photo, 107 men from the 1/4th King's Own still lie there - never found or identified after the war.