Slump disheartened, facing heavy defeat, but shoot (6)
I believe the answer is:
sprout
'shoot' is the definition.
('sprout' can be a synonym of 'shoot')
'slump disheartened facing heavy defeat' is the wordplay.
'disheartened' means to remove the middle letters.
'facing' is a charade indicator (letters next to each other).
'heavy defeat' becomes 'rout' (I've seen this before).
'slump' with its middle taken out is 'sp'.
'sp'+'rout'='SPROUT'
'but' is the link.
(Other definitions for sprout that I've seen before include "Grow during stupor" , "Push out new shoots" , "Begin to develop" , "something for Christmas dinner?" , "Start to grow" .)