Make a face, finally smashing vase (4)
I believe the answer is:
gurn
'make a face' is the definition.
(to gurn is to pull a funny or grotesque face)
'finally smashing vase' is the wordplay.
'finally' suggests the final letters.
'vase' becomes 'urn' (urn is a kind of vase).
The final letter of 'smashing' is 'g'.
'g'+'urn'='GURN'
(Other definitions for gurn that I've seen before include "Rung (anag.)" , "Create hideous mug" , "Make faces competitively" .)