Slim, stop crushing Rex! (8)
I believe the answer is:
decrease
'slim' is the definition.
(slim can mean to scale down or decrease)
'stop crushing rex' is the wordplay.
'stop' becomes 'decease' (to decease is to die or stop).
'crushing' is an insertion indicator (inserted letters are crushed by others).
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'decease' enclosing 'r' is 'DECREASE'.
(Other definitions for decrease that I've seen before include "Reduce in size" , "Get less" , "Dock" , "Deminish, lessen" , "Plummet" .)