Corpulent old boy seen mostly in disarray (5)
I believe the answer is:
obese
'corpulent' is the definition.
('obese' can be a synonym of 'corpulent')
'old boy seen mostly in disarray' is the wordplay.
'old boy' becomes 'ob'.
'mostly' means to remove the last letter.
'in disarray' indicates an anagram.
'seen' with its last letter taken away is 'see'.
'see' with letters rearranged gives 'ese'.
'ob'+'ese'='OBESE'
(Other definitions for obese that I've seen before include "getting well-upholstered" , "carrying remarkable weight" , "Roly-poly" , "Unhealthily large" , "Carrying far too much weight" .)