Stiff wind is a serious setback (4,4)
'a serious setback' is the definition.
(I've seen this before)
'stiff wind' is the wordplay.
'stiff' becomes 'body' (I've seen this before).
'wind' becomes 'blow' (blow is a kind of wind).
'body'+'blow'='BODY BLOW'
'is' acts as a link.
(Other definitions for body blow that I've seen before include "Crushing reverse" , "Huge setback" , "Severe disappointment" , "Severe setback, like thump to torso" , "Serious setback" .)
'stiff' is the definition.
(I've seen this before)
I can't explain the rest of the clue.
(Other definitions for body that I've seen before include "Full flavour in wine" , "Group of people" , "Corpse; full quality of wine" , "Corpse; full flavour" , "(Physical part of) person (or persons)" .)