Labourer against entering one of the armed forces (5)
I believe the answer is:
navvy
'labourer' is the definition.
(I know that navvy is a type of labourer)
'against entering one of the armed forces' is the wordplay.
'against' becomes 'v' (abbreviation for 'versus').
'entering' means one lot of letters goes inside another.
'one of the armed forces' becomes 'navy' (Royal Navy).
'v' placed within 'navy' is 'NAVVY'.
(Other definitions for navvy that I've seen before include "Canal constructor" , "Construction worker" , "Labourer, originally digging the canals" , "Construction labourer" , "Canal labourer" .)