Encourage sudden growth? Not entirely (4)
I believe the answer is:
spur
'encourage' is the definition.
(spur can mean to encourage)
'sudden growth? not entirely' is the wordplay.
'sudden growth?' becomes 'spurt' (as in a growth spurt).
'not entirely' means to remove the last letter.
'spurt' with its final letter taken off is 'SPUR'.
(Other definitions for spur that I've seen before include "Drive on" , "Goad - incentive" , "side road" , "Goad - branch line" , "Sharply pointed projection" .)