Need to head off social worker (3)
I believe the answer is:
ant
'social worker' is the definition.
(ants are social insects)
'need to head off' is the wordplay.
'need' becomes 'want' (want is a kind of need).
'to head off' means to remove the first letter (the first letter or 'head' is removed).
'want' with its first letter removed is 'ANT'.
(Other definitions for ant that I've seen before include "The insect lives in a social colony" , "A soldier, for one" , "Industrious insect" , "Dec's long-time partner" , "that may be a soldier" .)