Worst gemstones take in female bosom friends (4,2,4)

I believe the answer is:

best of pals

'bosom friends' is the definition.

'worst gemstones take in female' is the wordplay.
'worst' becomes 'best' (both can mean to beat someone).
'gemstones' becomes 'opals' (opal is a kind of gemstone).
'take in' means one lot of letters goes inside another.
'female' becomes 'f' (abbreviation).
'opals' placed around 'f' is 'ofpals'.
'best'+'ofpals'='BEST OF PALS'

I've seen this clue in The Independent.

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