Corpulent woman's old man (6)
I believe the answer is:
father
'old man' is the definition.
(I've seen this before)
'corpulent woman's' is the wordplay.
'corpulent' becomes 'fat' ('fat' can be similar in meaning to 'corpulent').
'woman' becomes 'her' (**).
'fat'+'her'='FATHER'
(Other definitions for father that I've seen before include "Patriarch" , "The - of the House is the longest-serving MP" , "See 1 Across" , "Originate; priest" , "PA" .)