Octogenarian is this heavy, not wife (6)
I believe the answer is:
eighty
'octogenarian' is the definition.
(thesaurus)
'this heavy not wife' is the wordplay.
'this heavy' becomes 'weighty' (I've seen this before).
'not' is a deletion indicator.
'wife' becomes 'w'.
'weighty' with 'w' taken away is 'EIGHTY'.
'is' is the link.
(Other definitions for eighty that I've seen before include "not many score" , "Fourscore" , "Northern county (abbr.)" , "Around the World in - Days (Verne)" , "Four score" .)