Outgrowth caused by parasite in cheek (4)
I believe the answer is:
I believe this is a double definition.
'outgrowth caused by parasite' is the first definition.
(parasitic growth on a plant)
'cheek' is the second definition.
(both can mean outrageous behaviour)
'in' is the link.
(Other definitions for gall that I've seen before include "Impudence, cheek" , "Impertinence" , "Impudence; bitterness" , "This bladder is attached to the liver and stores bile" , "Growth on tree; bile; resentment" .)