At first, during sex, one is accompanied by partner (9)

I believe the answer is:

initially

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'at first' is the definition.
(synonyms)

'during sex one is accompanied by partner' is the wordplay.
'during' becomes 'in' (e.g. during February means in February).
'sex' becomes 'it' (common euphemism).
'one' becomes 'i' (Roman numeral).
'is accompanied by partner' becomes 'ally' (I can't explain this - if you can you should give a lot more credence to this answer).
'in'+'it'+'i'+'ally'='INITIALLY'

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(Other definitions for initially that I've seen before include "At first" , "To begin with" , "At the start" , "In the beginning" , "Firstly" .)

I've seen this clue in The Independent.

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