Never has waist doubled? Not that old! (5)
I believe the answer is:
newer
'not that old' is the definition.
'never has waist' is the wordplay.
'never' becomes 'neer' ('ne'er' can be a synonym of 'never').
'has' indicates putting letters inside.
'waist' becomes 'w' (this could be a standard abbreviation which I don't know about).
'neer' placed around 'w' is 'NEWER'.
'doubled?' acts as a link.
I am not very happy about this link. It may belong to another bit of the clue.
(Other definitions for newer that I've seen before include "More recent" , "More pristine or recently invented" , "More up-to-date" , "More modern" , "Fresher, not as old" .)