Sent a fun cocktail to loosen them up (8)

I believe the answer is:

unfasten

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'loosen them up' is the definition.
I can't tell whether this definition defines the answer.

'sent a fun cocktail' is the wordplay.
'cocktail' indicates an anagram.
'sent'+'a'+'fun'='sentafun'
'sentafun' with letters rearranged gives 'UNFASTEN'.

'to' is the link.

Can you help me to learn more?

(Other definitions for unfasten that I've seen before include "Undo" , "tie? No" , "Let go" , "Unlock" , "Nun's fate (anag.)" .)

I've seen this clue in The Irish Times.

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