Caught rule on nail (4)
I believe the answer is:
claw
'nail' is the definition.
(I've seen this in another clue)
'caught rule on' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'rule' becomes 'law' (law is a kind of rule).
'on' says to put letters next to each other.
'c'+'law'='CLAW'
(Other definitions for claw that I've seen before include "Scratch - scrape" , "Bird's foot" , "See 3" , "Hook on the end of toe" , "See 24" .)