Keen teenager's outside entrance (9)
I believe the answer is:
trenchant
'keen' is the definition.
(synonyms)
'teenager's outside entrance' is the wordplay.
'outside' says to hollow out the word (remove centre letters).
'entrance' becomes 'enchant' (to entrance is to enchant or bewitch).
'teenager' with its middle removed is 'tr'.
'tr'+'enchant'='TRENCHANT'
(Other definitions for trenchant that I've seen before include "blunt" , "Distinct" , "Keen" , "Forthright, vigorous" , "Clear-cut and hard-hitting" .)