Refrain from smiling (4,1,8,4)
I believe the answer is:
keep a straight face
'refrain from smiling' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for keep a straight face that I've seen before include "Don't corpse" , "give nothing away" , "Refrain from smiling" , "Manage to avoid laughing" , "appear serious" .)