Downside to severe cramp (9)
I believe the answer is:
constrict
'cramp' is the definition.
(I've seen this before)
'downside to severe' is the wordplay.
'downside' becomes 'con' (I've seen this in another clue).
'to severe' becomes 'strict' (I've seen this in another clue).
'con'+'strict'='CONSTRICT'
(Other definitions for constrict that I've seen before include "Strangle" , "Strangulate" , "Make narrower" , "Inhibit" , "press" .)