Why union’s not gone ahead with opening of semipro arrangement (6,2,7)
I believe the answer is:
breach of promise
'why union's not gone ahead' is the definition.
'opening of semipro arrangement' is the wordplay.
'opening' becomes 'breach' (breach is a kind of opening**).
'arrangement' indicates anagramming the letters.
'semipro' anagrammed gives 'promise'.
'breach'+'of'+'promise'='BREACH OF PROMISE'
'with' is the link.
(Other definitions for breach of promise that I've seen before include "Suit" , "failing to meet expectations?" , "a danger when lover gets overcommitted?" , "end of engagement" , "failure to fulfil an engagement?" .)