Willow is more promising without top (5)
I believe the answer is:
osier
'willow' is the definition.
(I know that osier is a type of willow)
'more promising without top' is the wordplay.
'more promising' becomes 'rosier' ('rosy' can be a synonym of 'promising').
'without top' means to remove the first letter.
'rosier' with its initial letter taken away is 'OSIER'.
'is' acts as a link.
(Other definitions for osier that I've seen before include "Withy" , "Tree - Rosie (anag)" , "growing in a bed" , "Basket-maker" , "one used to get weaving?" .)