Social worker takes one to be an opponent (4)
I believe the answer is:
anti
'opponent' is the definition.
(I've seen this before)
'social worker takes one' is the wordplay.
'social worker' becomes 'ant' (I've seen this before**).
'takes' means one lot of letters go next to another.
'one' becomes 'i' (Roman numeral).
'ant'+'i'='ANTI'
'to be an' acts as a link.
(Other definitions for anti that I've seen before include "not favoured" , "An opponent - opposed to party policy, etc" , "[NO MAN]" , "Policy's opponent" , "Adversary" .)