Sulphur found in old beer, mostly stout (5)
I believe the answer is:
obese
'stout' is the definition.
(I've seen this before)
'sulphur found in old beer mostly' is the wordplay.
'sulphur' becomes 's' ('S' can be a synonym of 'sulphur').
'found in' means one lot of letters goes inside another.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'mostly' means to remove the last letter.
'o'+'beer'='obeer'
'obeer' with its last letter removed is 'obee'.
's' going within 'obee' is 'OBESE'.
(Other definitions for obese that I've seen before include "Clinically overweight" , "Very big" , "More than podgy" , "having generous corporation" , "Extremely portly" .)