Humming after crossword essentially well-constructed (5)
I believe the answer is:
solid
'well-constructed' is the definition.
(thesaurus)
'humming after crossword essentially' is the wordplay.
'humming' becomes 'olid' (both can mean smelly).
'after' says to put letters next to each other.
'essentially' indicates the central letters.
The centre of 'crossword' is 's'.
'olid' put after 's' is 'SOLID'.
(Other definitions for solid that I've seen before include "Sound; substantial" , "Firm; not liquid" , "Unbroken" , "Idols (anag.)" , "Firmly constructed" .)