Son filling belly with old relish (5)
I believe the answer is:
gusto
'relish' is the definition.
('gusto' can be a synonym of 'relish')
'son filling belly with old' is the wordplay.
'son' becomes 's'.
'filling' means one lot of letters goes inside another.
'belly' becomes 'gut' (I've seen this before).
'with' means one lot of letters go next to another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
's' inserted inside 'gut' is 'gust'.
'gust'+'o'='GUSTO'
(Other definitions for gusto that I've seen before include "Energy" , "Great enjoyment, vigour" , "Great relish" , "Relish - zest" , "Zest, exuberance" .)