Condition favouring six to such an extent (7)
I believe the answer is:
proviso
'condition' is the definition.
(proviso is a kind of condition)
'favouring six to such an extent' is the wordplay.
'favouring' becomes 'pro' (I've seen this in other clues).
'six' becomes 'vi' ('vi' can be a synonym of 'six').
'to such an extent' becomes 'so'.
'pro'+'vi'+'so'='PROVISO'
(Other definitions for proviso that I've seen before include "Condition for VIP or so" , "Stipulated condition" , "Requirement" , "Condition of stipulation of an agreement" , "Clause in contract" .)