Accident puts Rex in pain (5)
I believe the answer is:
prang
'accident' is the definition.
(I've seen this before)
'rex in pain' is the wordplay.
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'in' is an insertion indicator.
'pain' becomes 'pang' (pang is a kind of pain**).
'r' going within 'pang' is 'PRANG'.
'puts' is the link.
(Other definitions for prang that I've seen before include "knock" , "Car accident, usually not too serious" , "Popularly, a motoring contretemps" , "Minor shunt" , "Bomb attack" .)