Opposed to introduction of animal trap (4)
I believe the answer is:
agin
'opposed to' is the definition.
(I've seen this before)
'introduction of animal trap' is the wordplay.
'introduction of' indicates taking the first letters.
'trap' becomes 'gin' (I've seen this before).
The first letter of 'animal' is 'a'.
'a'+'gin'='AGIN'
(Other definitions for agin that I've seen before include "Hostile to" , "Dialect form of against" , "Not a fan of" , "Against (facetious)" , "Not in favour of" .)