Piled it on when he did an impression (6)

I believe the answer is:

heaped

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'piled' is the definition.
('heap' can be a synonym of 'pile')

'he did an impression' is the wordplay.
'did an impression' becomes 'aped' (I've seen this in another clue).
'he'+'aped'='HEAPED'

'it on when' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.

Can you help me to learn more?

(Other definitions for heaped that I've seen before include "Fully laden" , "made a pile" , "Piled, stacked" .)

I've seen this clue in the Evening Standard.

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