Sir's tum is suffering a spasm (7)
I believe the answer is:
trismus
'a spasm' is the definition.
(thesaurus)
'sir's tum is suffering' is the wordplay.
'is suffering' indicates an anagram.
'sirs'+'tum'='sirstum'
'sirstum' with letters rearranged gives 'TRISMUS'.
I've seen this clue in The Telegraph.
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