Prevent animal swallowing the tin opener (5)
I believe the answer is:
deter
'prevent' is the definition.
(I've seen this before)
'animal swallowing the tin opener' is the wordplay.
'animal' becomes 'deer' (deer is a type of animal).
'swallowing' indicates putting letters inside.
'opener' indicates taking the first letters.
The initial letter of 'tin' is 't'.
'deer' placed around 't' is 'DETER'.
(Other definitions for deter that I've seen before include "Dishearten" , "stop" , "Inhibit" , "Check" , "Discourage, put off" .)