Corpulent southern racecourse spy (5)
I believe the answer is:
stout
'corpulent' is the definition.
(I've seen this before)
'southern racecourse spy' is the wordplay.
'southern' becomes 's' (abbreviation).
'racecourse spy' becomes 'tout' (gambler who scouts out a race course).
's'+'tout'='STOUT'
(Other definitions for stout that I've seen before include "of above-average weight" , "Inured to fatigue and hardship" , "Bulky" , "Rather fat; determined" , "Rather fat, drink" .)