Plant needs a drink (4)
I believe the answer is:
arum
'plant' is the definition.
(arum is a kind of plant)
'a drink' is the wordplay.
'drink' becomes 'rum' (I've seen this before).
'a'+'rum'='ARUM'
'needs' is the link.
(Other definitions for arum that I've seen before include "Plant with small flowers and arrow-shaped leaves - kind of lily" , "Plant of the family Araceae, eg lords-and-ladies" , "Cuckoo-pint" , "plant with a white spathe" , "A sort of lily." .)