Police seizing weapons used for bagging old animal (8)
I believe the answer is:
marmoset
'animal' is the definition.
(I've seen this before)
'police seizing weapons used for bagging old' is the wordplay.
I cannot quite understand how this works, but
'old' could be 'o' (abbreviation as in OT - Old Testament) and 'o' is found within the answer.
'weapons' could be 'arms' ('arm' can be a synonym of 'weapon') and 'arms' is found in the remaining letters.
'police' could be 'met' (London Metropolitan Police) and 'met' is found within the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for marmoset that I've seen before include "Turn morse mat over for the monkey" , "Small tropical American monkey" , "Little monkey" , "Star memo about kind of monkey" , "Take me to arms about bushy-tailed monkey of tropical America" .)