There's nothing in beer but a plant (4)
I believe the answer is:
aloe
'a plant' is the definition.
(aloe is a kind of plant)
'there's nothing in beer' is the wordplay.
'there's nothing' becomes 'o' (I've seen this in other clues).
'in' indicates putting letters inside.
'beer' becomes 'ale' (type of beer).
'o' going into 'ale' is 'ALOE'.
'but' acts as a link.
(Other definitions for aloe that I've seen before include "Flower, maybe vera" , "O, ale with bitter plant" , "Succulent plant (may be bitter!)" , "shrub" , "Fleshy-leaved plant" .)