Claw, in fact a long one (5)
I believe the answer is:
talon
'claw' is the definition.
(I know that talon is a type of claw)
'in fact a long one' is the wordplay.
'in' indicates a hidden word.
'TALON' is hidden in the letters of 'fact a long one'.
(Other definitions for talon that I've seen before include "Claw of a bird of prey" , "One hooked on bird" , "Large claw" , "One may be grabbing" , "Hooked claw; cards left after deal" .)