Nail not good when pinning end of finger (4)
I believe the answer is:
'nail' is the definition.
(a brad is a nail with rectangular cross-section)
'not good when pinning end of finger' is the wordplay.
'not good' becomes 'bad' (bad is the opposite of good).
'when pinning' means one lot of letters goes inside another.
'end of' suggests the final letters.
The final letter of 'finger' is 'r'.
'bad' going around 'r' is 'BRAD'.
(Other definitions for brad that I've seen before include "One may secure" , "Fixer" , "Small-headed nail" , "It has a point" , "Small tapered nail" .)