Unreasonable move involving Ecstasy (5)
I believe the answer is:
steep
'unreasonable' is the definition.
(thesaurus)
'move involving ecstasy' is the wordplay.
'move' becomes 'step' (step is a kind of move**).
'involving' indicates putting letters inside.
'ecstasy' becomes 'e' (informal term for the drug).
'step' placed around 'e' is 'STEEP'.
(Other definitions for steep that I've seen before include "Without the bounds of moderation" , "Rapidly rising" , "Having a sharp incline" , "Rising sharply" , "Vertiginous; excessive" .)