Extremely fashionable viewed in mirror, almost entirely (5,4)
I believe the answer is:
every inch
'entirely' is the definition.
(I've seen this before)
'extremely fashionable viewed in mirror almost' is the wordplay.
'extremely' becomes 'very' (I've seen this before).
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'viewed in' means one lot of letters goes inside another.
'mirror' becomes 'echo'.
'almost' means to remove the last letter (most of the word but not all of it).
'very'+'in'='veryin'
'echo' with its final letter removed is 'ech'.
'veryin' going within 'ech' is 'EVERY INCH'.
(Other definitions for every inch that I've seen before include "In all departments" , "completely" , "Wholly" , "Absolutely!" , "Entirely" .)