Best friend will eat one cold, in current fashion (9)
I believe the answer is:
topically
'in current fashion' is the definition.
(I've seen this before)
'best friend will eat one cold' is the wordplay.
'best' becomes 'top' ('top' can be similar in meaning to 'best').
'friend' becomes 'ally' (I've seen this before).
'will eat' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'cold' becomes 'c' (eg on taps).
'top'+'ally'='topally'
'i'+'c'='ic'
'topally' enclosing 'ic' is 'TOPICALLY'.
(Other definitions for topically that I've seen before include "in current fashion" , "contemporary way" .)