Pair from union in function (3)
I believe the answer is:
duo
'pair' is the definition.
('duo' can be a synonym of 'pair')
'union in function' is the wordplay.
'union' becomes 'u' (common abbreviation as in 'Trades Union Congress').
'in' means one lot of letters goes inside another.
'function' becomes 'do' (I've seen this in other clues).
'u' going inside 'do' is 'DUO'.
'from' is the link.
(Other definitions for duo that I've seen before include "Two of a kind" , "Pair of performers" , "Item" , "Pair of musicians" , "rejected Arab's lute" .)