Wife assumes husband's role in store to barter (4)

I believe the answer is:

swop

I'm a little stuck... Click here to teach me more about this clue!

'barter' is the definition.
(synonyms)

'wife assumes husband's role in store' is the wordplay.
I cannot quite understand how this works, but
'wife' could be 'w' and 'w' is located in the answer.
The remaining letters 'sop' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...

'to' is the link.

Can you help me to learn more?

(Another definition for swop that I've seen is " Exchange (Alt sp.)".)

I've seen this clue in The Telegraph.

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