Playing heavily on pretence (10)

I believe the answer is:

overacting

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'playing' is the definition.
(I know that overacting is a type of playing)

'heavily on pretence' is the wordplay.
'heavily' becomes 'overing' (I can't justify this - if you can you should give a lot more credence to this answer).
'on' indicates putting letters inside (as in clothing 'on' a person).
'pretence' becomes 'act'.
'overing' going around 'act' is 'OVERACTING'.

Can you help me to learn more?

(Another definition for overacting that I've seen is " Exaggeration in Whitehall, say".)

I've seen this clue in the Sydney Morning Herald.

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