Act decently in drama festival (8)

I believe the answer is:

playfair

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'drama festival' is the definition.
The definition suggests a singular noun which matches the answer.

'act decently' is the wordplay.
'act' becomes 'play' (play is a kind of act).
'decently' becomes 'fair' (I can't justify this - if you can you should believe this answer much more).
'play'+'fair'='PLAYFAIR'

'in' acts as a link.

Can you help me to learn more?

(Other definitions for playfair that I've seen before include "Decline to cheat" , "Don't cheat" , "Act honestly" .)

I've seen this clue in The Times.

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