Gym culture so unorthodox it produces dangerous opponent (4,8)

I believe the answer is:

ugly customer

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'it produces dangerous opponent' is the definition.
The definition suggests a singular noun which matches the answer.

'gym culture so unorthodox' is the wordplay.
'unorthodox' indicates an anagram.
'gym'+'culture'+'so'='gymcultureso'
'gymcultureso' with letters rearranged gives 'UGLY CUSTOMER'.

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(Other definitions for ugly customer that I've seen before include "Dangerous antagonist" , "Thug" , "viewed unfairly by shopkeeper?" , "Hard case" .)

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