Pop back to fix (3)

I believe the answer is:

'fix' is the definition.
(to pin is to fix in place)

'pop back' is the wordplay.
'pop' becomes 'nip' ('nip out' and 'pop out' can both mean to go out).
'back' is a reversal indicator.
'nip' in reverse letter order is 'PIN'.

'to' acts as a link.

(Other definitions for pin that I've seen before include "Fasten; a security code" , "leg" , "Fastener; brooch" , "Stick with flag placed in golf hole" , "Small brooch; skittle" .)

I've seen this clue in the Evening Standard.

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