Carry out, when through the questionnaire (7)

I believe the answer is:

perform

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'carry out' is the definition.
(performing is a kind of carrying out)

'through the questionnaire' is the wordplay.
'through' becomes 'per' (I've seen this before).
'the questionnaire' becomes 'form' (I can't explain this - if you can you should give a lot more credence to this answer).
'per'+'form'='PERFORM'

'when' is the link.

Can you help me to learn more?

(Other definitions for perform that I've seen before include "function" , "Carry out; act" , "Carry out (action)" , "Act in play or film" , "act on stage" .)

I've seen this clue in the Evening Standard.

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