Former sweetheart, playing hard ... (8)

I believe the answer is:

exacting

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'hard' is the definition.
(thesaurus)

'former sweetheart playing' is the wordplay.
'former' becomes 'ex' (prefix meaning former or onetime).
'sweetheart playing' becomes 'acting' (I can't explain this - if you can you should believe this answer much more).
'ex'+'acting'='EXACTING'

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(Other definitions for exacting that I've seen before include "Meticulous" , "Requiring hard work and attention" , "Challenging" , "Strict, demanding" , "Requiring great effort, arduous" .)

I've seen this clue in The Telegraph.

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