"Fireman " was he a fireman on the railway - or with the fire service - or even a fireman on a ship (stoker )??
trying to establish why he was in Copenhagen ??...the obvious is a ships fireman - being Copenhagen is a large sea port
on the 1881 census does his occupation just say " fireman " ??
ADDED ...is this the marriage ?
13th May 1876 St John Preston
Arthur John Clegg .. bachelor aged 23 Labourer abode 3 Carroll Street
Betsy Blackburn - spinster aged 22 Weaver 3 Slater Street (x)
Grooms father the late Joseph Clegg an Artist
Brides father Richard Blackburn a Spinner
witness Thomas Cocker Margaret Cocker
Married by Banns William Ritson
Notes - Groom signed request
?...what does this mean ?...never seen it before
Banns: St John, Preston, Lancashire, England
Arthur John Clegg - 22, 3 Carroll Street
Betsy Blackburn - 22, 3 Slater Street
Notes: [Groom signed request]
Groom Residence: 2 years
Bride Residence: 2 months
ADDED
They are in the mothers house in 3 Carroll Street in 1881 census ...
just says "fireman"
https://familysearch.org/pal:/MM9.1.1/XQ66-NN6?cc=1321821out of curiosity - Joseph Clegg - Arthurs father - on the marriage record he is an Artist ..do you have him in an earlier census in Preston ? 1871 ?....Arthur would be around 17 ..may give a clue as to his occupation ..
added again ...doesn`t give occupations
https://familysearch.org/pal:/MM9.1.1/KZPL-68R