Stop Des first! (6)
I believe the answer is:
desist
'stop' is the definition.
(thesaurus)
'des first' is the wordplay.
'first' becomes 'ist'.
'des'+'ist'='DESIST'
(Other definitions for desist that I've seen before include "Stop, cease activity" , "Cease; abstain" , "Stop, discontinue" , "Refrain (from doing something)" , "Forbear" .)