Big cat finally running inside bank (5)
I believe the answer is:
tiger
'big cat' is the definition.
(I know that tiger is a type of big cat)
'finally running inside bank' is the wordplay.
'finally' suggests the final letters.
'inside' indicates putting letters inside.
'bank' becomes 'tier' (both can mean a row of similar things).
The final letter of 'running' is 'g'.
'g' going within 'tier' is 'TIGER'.
(Other definitions for tiger that I've seen before include "Fierce person" , "See 13" , "Large cat with black stripes" , "Dynamo" , "Dangerous animal" .)