They smell new -- flowers pruned at the top (5)
I believe the answer is:
noses
'they smell' is the definition.
(noses smell things)
'new flowers pruned at the top' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'flowers' becomes 'roses' (I've seen this before).
'pruned at the top' means to remove the first letter (the first letter is pruned off).
'roses' with its first letter removed is 'oses'.
'n'+'oses'='NOSES'
(Other definitions for noses that I've seen before include "Olfactory organs" , "Snouts" , "Facial features" , "Hooters" , "Organs of smell" .)