Suffering one back spasm is wrong (7)
I believe the answer is:
illicit
'is wrong' is the definition.
(I've seen this before)
'suffering one back spasm' is the wordplay.
'suffering' becomes 'ill' (suffering with illness).
'one' becomes 'I' (Roman numeral).
'back' is a reversal indicator.
'spasm' becomes 'tic' (tic is a kind of spasm).
'tic' written backwards gives 'cit'.
'ill'+'i'+'cit'='ILLICIT'
(Other definitions for illicit that I've seen before include "Not approved by law" , "not conforming to regulations" , "Without approval" , "Forbidden by law, rules" , "breaking the law" .)