Many a chap, should he be getting on in years (8)
I believe the answer is:
manifold
'many' is the definition.
(thesaurus)
'a chap should he be getting on in years' is the wordplay.
'a chap' becomes 'man' (chap can mean a man).
'should he' becomes 'if' (I can't explain this - if you can you should give a lot more credence to this answer).
'be getting on in years' becomes 'old' (getting older).
'man'+'if'+'old'='MANIFOLD'
Can you help me to learn more?
(Other definitions for manifold that I've seen before include "Abundant" , "Many and varied" , "Chamber" , "In various ways" , "car part?" .)