Superman's pronounced cramp (7)

I believe the answer is:

tighten

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'cramp' is the definition.
(thesaurus)

I cannot understand how the remainder of the clue works.

(Other definitions for tighten that I've seen before include "Give an extra turn" , "Make tauter" , "Do this to your belt to economise (7)" , "Make less loose" , "To improve focus" .)

I've seen this clue in The Sun.

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