A social worker I opposed (4)
I believe the answer is:
anti
'opposed' is the definition.
(similar in meaning)
'a social worker i' is the wordplay.
'a social worker' becomes 'ant' (ants are social insects).
'ant'+'i'='ANTI'
(Other definitions for anti that I've seen before include "Hostile" , "Prefix meaning against" , "Not favouring" , "Opposed to followers of Hitler" , "Who pooh-poohs all" .)