One against one backing worker (4)
I believe the answer is:
anti
'one against' is the definition.
(I've seen this before)
'one backing worker' is the wordplay.
'one' becomes 'i' (Roman numeral).
'backing' says to put letters next to each other.
'worker' becomes 'ant' (worker is a type of ant**).
'i' put after 'ant' is 'ANTI'.
(Other definitions for anti that I've seen before include "Policy's opponent" , "one opposing?" , "Prefix meaniing against" , "Negative vote" , "Definitely not pro" .)