Put on a long face (4)
I believe the answer is:
mope
'put on a long face' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for mope that I've seen before include "Languish" , "Feel dejected and listless" , "Be listless or depressed" , "Yield to low spirits with an odd poem" , "Move listlessly and aimlessly" .)