Stage influence is joke (3-4)
I believe the answer is:
leg-pull
'is joke' is the definition.
(I've seen this before)
'stage influence' is the wordplay.
'stage' becomes 'leg' (synonyms).
'influence' becomes 'pull' (I've seen this before).
'leg'+'pull'='LEG-PULL'
(Other definitions for leg-pull that I've seen before include "Good-humoured hoax" , "Trick; practical joke" , "Informal hoax involving lower limb" , "Hoax exerted on lower limb?" , "Trying to make somebody believe what is not true as a joke" .)