Smart support in back of hostelry (6)
I believe the answer is:
brainy
'smart' is the definition.
(thesaurus)
'support in back of hostelry' is the wordplay.
'support' becomes 'bra' (a support for the breasts).
'back of' says to take the final letters.
The last letter of 'hostelry' is 'y'.
'bra'+'in'+'y'='BRAINY'
(Other definitions for brainy that I've seen before include "Intellectual" , "Bright" , "smart?" , "Intelligent, clever." , "Showing intelligence" .)