Not totally against insect (3)
I believe the answer is:
ant
'insect' is the definition.
(ant is a kind of insect)
'not totally against' is the wordplay.
'not totally' means to remove the last letter.
'against' becomes 'anti' (prefix meaning against).
'anti' with its last letter removed is 'ANT'.
(Other definitions for ant that I've seen before include "Colonial insect?" , "Small social insect" , "Worker on the hill" , "One more provident than Grasshopper" , "Dec's long-time partner" .)