Most unexpectedly opposing everything offered by bookmaker (7,3,4)
I believe the answer is:
against all odds
'most unexpectedly' is the definition.
'opposing everything offered by bookmaker' is the wordplay.
'opposing' becomes 'against' (I've seen this before).
'everything' becomes 'all' (I've seen this before).
'offered by bookmaker' becomes 'odds'.
'against'+'all'+'odds'='AGAINST ALL ODDS'
(Other definitions for against all odds that I've seen before include "Most improbably" , "favouring 2,4,6,8, as you might suppose?" .)