Legend behind curmudgeon (5)
I believe the answer is:
grump
'curmudgeon' is the definition.
(I've seen this in another clue)
'legend behind' is the wordplay.
'legend' becomes 'g' (leg-end - end letter of 'leg').
'behind' becomes 'rump' (informal term for the buttocks).
'g'+'rump'='GRUMP'
(Other definitions for grump that I've seen before include "Bad-tempered person" , "Surly person" , "Crosspatch" , "grouse" , "Beef" .)