Rat briefly cuts supply in plant (6)
I believe the answer is:
cactus
'plant' is the definition.
(cactus is a kind of plant)
'rat briefly cuts supply' is the wordplay.
'rat' becomes 'cad' (I've seen this in other clues).
'briefly' means to remove the last letter.
'supply' indicates anagramming the letters (supply of letters to make).
'cad' with its final letter taken off is 'ca'.
'cuts' anagrammed gives 'ctus'.
'ca'+'ctus'='CACTUS'
'in' acts as a link.
(Other definitions for cactus that I've seen before include "Thorny desert plant" , "plant, succulent one" , "Spiney plant of dry areas" , "Desert flower" , "Spiny plant of dry regions" .)