Twelve from Australia engaged in study (5)
I believe the answer is:
dozen
'twelve' is the definition.
(synonyms)
'australia engaged in study' is the wordplay.
'australia' becomes 'oz' (informal name for Australia).
'engaged in' means one lot of letters goes inside another.
'study' becomes 'den' (term for a study or office).
'oz' placed into 'den' is 'DOZEN'.
'from' is the link.
(Other definitions for dozen that I've seen before include "Twelfth of a gross" , "See 4" , "Set of 12" , "Quantity of e.g. eggs" , "Set of twelve" .)