The social worker with a good book (3)
I believe the answer is:
ant
'the social worker' is the definition.
(ants are social insects)
'with a good book' is the wordplay.
'with' says to put letters next to each other.
'good book' becomes 'NT' (New Testament).
'a' next to 'nt' is 'ANT'.
(Other definitions for ant that I've seen before include "Social worker" , "one living in colonies" , "Dec's long-time partner" , "Emmet or pismire" , "Worker on the hill" .)