Old friend purchases outfit, fashionable at first (10)
I believe the answer is:
originally
'at first' is the definition.
'old friend purchases outfit fashionable' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'friend' becomes 'ally' (I've seen this before).
'purchases' means one lot of letters goes inside another.
'outfit' becomes 'rig' ('rig' can be a synonym of 'outfit'**).
'fashionable' becomes 'in' ('in' can be similar in meaning to 'fashionable').
'o'+'ally'='oally'
'rig'+'in'='rigin'
'oally' going around 'rigin' is 'ORIGINALLY'.
(Other definitions for originally that I've seen before include "to begin with" , "In the first place" , "new way" , "At the start" .)