Former lover criticises one on increase in size (9)
I believe the answer is:
expansion
'increase in size' is the definition.
'former lover criticises one on' is the wordplay.
'former lover' becomes 'ex' (an ex-lover).
'criticises' becomes 'pans' (I've seen this before).
'one' becomes 'i' (Roman numeral).
'ex'+'pans'+'i'+'on'='EXPANSION'
(Other definitions for expansion that I've seen before include "increase in capacity" , "Process of increasing in size, volume etc" , "Enlargement" , "fatter" , "Increase in volume" .)