Social worker one opposed (4)
I believe the answer is:
anti
'opposed' is the definition.
(similar in meaning)
'social worker one' is the wordplay.
'social worker' becomes 'ant' (ants are social insects).
'one' becomes 'i' (Roman numeral).
'ant'+'i'='ANTI'
(Other definitions for anti that I've seen before include "on the other side" , "not favoured" , "one opposing?" , "'No' voter" , "Not in favor of" .)