Biggest supporter in ground hit bouncer close to exit (7)
I believe the answer is:
taproot
'biggest supporter in ground' is the definition.
(large root in a plant)
'hit bouncer close to exit' is the wordplay.
'hit' becomes 'tap' (I've seen this before**).
'bouncer' becomes 'roo' (informal term for a kangaroo).
'close to' suggests the final letters (the close/ending of).
The last letter of 'exit' is 't'.
'tap'+'roo'+'t'='TAPROOT'
(Other definitions for taproot that I've seen before include "Main underground part of plant" , "Plant's main source of nourishment" , "Main part of plant in earth" , "Part of plant growing down vertically" , "infrastructure" .)