Worker is against wearing blue (5)
I believe the answer is:
navvy
'worker' is the definition.
(navvy is a kind of worker)
'against wearing blue' is the wordplay.
'against' becomes 'v' (abbreviation for 'versus').
'wearing' indicates putting letters inside.
'blue' becomes 'navy' (shade of blue).
'v' placed into 'navy' is 'NAVVY'.
'is' acts as a link.
(Other definitions for navvy that I've seen before include "Labourer working on canals, roads, railways etc?" , "Construction worker" , "Labourer, originally digging the canals" , "Canal constructor" , "Canal labourer" .)