Female inside definitely astray (3-6)

I believe the answer is:

off-course

'astray' is the definition.

'female inside definitely' is the wordplay.
'female' becomes 'f' (abbreviation).
'inside' is an insertion indicator.
'definitely' becomes 'of course' (both can mean 'yes').
'f' inserted inside 'ofcourse' is 'OFF COURSE'.

I've seen this clue in The Times.

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