Stimulus a Right Reverend will get in bed (6)
I believe the answer is:
carrot
'stimulus' is the definition.
(as in the carrot and stick method)
'a right reverend will get in bed' is the wordplay.
'right reverend' becomes 'rr' (abbreviation for Right Reverend).
'will get in' means one lot of letters goes inside another.
'bed' becomes 'cot' (cot is a kind of bed).
'a'+'rr'='arr'
'arr' inserted into 'cot' is 'CARROT'.
(Other definitions for carrot that I've seen before include "Red vegetable used as inducement" , "Obstacle in race" , "Pointed vegetable" , "One prov. used to encourage" , "Orangey vegetable" .)