To that extent fashionable up to now (7)
I believe the answer is:
insofar
'to that extent' is the definition.
('Insofar' can be a synonym of 'to that extent')
'fashionable up to now' is the wordplay.
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'up to now' becomes 'sofar' (I've seen this before).
'in'+'sofar'='INSOFAR'
(Other definitions for insofar that I've seen before include "... to the extent" , "as you'd expect after this word's been given" .)