Labourer against boarding ships (5)
I believe the answer is:
navvy
'labourer' is the definition.
(I know that navvy is a type of labourer)
'against boarding ships' is the wordplay.
'against' becomes 'v' (versus).
'boarding' indicates putting letters inside.
'ships' becomes 'navy' (I've seen this before).
'v' put inside 'navy' is 'NAVVY'.
(Other definitions for navvy that I've seen before include "Canal constructor" , "Labourer, originally digging the canals" , "Canal labourer" , "Construction worker" , "Construction labourer" .)