Bridge rivals addressed by international partner, primarily (11)
I believe the answer is:
essentially
'primarily' is the definition.
(I've seen this before)
'bridge rivals addressed by international partner' is the wordplay.
'bridge rivals' becomes 'ES' ('East' and 'South' are rivals in the game bridge).
'addressed' becomes 'sent' (I've seen this in other clues).
'by' means one lot of letters go next to another.
'international' becomes 'i' (abbreviation eg in IMF).
'partner' becomes 'ally' (I've seen this in other clues).
'es'+'sent'+'i'+'ally'='ESSENTIALLY'
(Other definitions for essentially that I've seen before include "Fundamentally" , "mainly" , "Basically" , "By its very nature" , "Primarily" .)