Crossword Genius

Folly to finish back in service (7)

Ross

I believe the answer is:

madness

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'folly' is the definition.
(thesaurus)

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(Other definitions for madness that I've seen before include "Group" , "Insanity" , "Irrationality" , "Lunacy - ska/pop band" , "Dementia" .)

I've seen this clue in The Times.
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