Fat old boy eats sweets eagerly at first (5)
I believe the answer is:
obese
'fat' is the definition.
('obese' can be similar in meaning to 'fat')
'old boy eats sweets eagerly at first' is the wordplay.
'old boy' becomes 'ob'.
'at first' suggests taking the first letters.
The first letters of 'eats sweets eagerly' is 'ese'.
'ob'+'ese'='OBESE'
(Other definitions for obese that I've seen before include "Unusually fat" , "Having excessive girth" , "having generous corporation" , "Roly-poly" , "likely to tip the balance?" .)