Former student given finally more Guinness double stout (5)
I believe the answer is:
obese
'stout' is the definition.
(both can mean overweight)
'former student given finally more guinness double' is the wordplay.
'former student' becomes 'OB' (abbreviation for 'Old Boy').
'given' is a charade indicator (letters next to each other).
'finally' suggests the final letters.
The final letters of 'more guinness double' are 'ese'.
'ob'+'ese'='OBESE'
(Other definitions for obese that I've seen before include "Very substantial" , "More than podgy" , "Unhealthily fat and overweight" , "Morbidly fat" , "Having excessive girth" .)