Push from United not in vain (4)
I believe the answer is:
prod
'push' is the definition.
(prodding is a kind of pushing)
'united not in vain' is the wordplay.
'united' becomes 'u'.
'not in' is a deletion indicator.
'vain' becomes 'proud' ('proud' can be similar in meaning to 'vain').
'proud' with 'u' taken out is 'PROD'.
'from' is the link.
(Other definitions for prod that I've seen before include "Poke (with a finger, etc)" , "Induce action (with stick etc?)" , "Drop (anag.)" , "Poke with a stick" , "Dig" .)