Appearance of fast current — no going back (10)
I believe the answer is:
expression
'appearance' is the definition.
(I know that expression is a type of countenance)
'fast current no going back' is the wordplay.
'fast' becomes 'express' (I've seen this before).
'current' becomes 'I' (symbol for electric current).
'going back' shows that the letters should be reversed in order.
'no' backwards is 'on'.
'express'+'i'+'on'='EXPRESSION'
'of' is the link.
(Other definitions for expression that I've seen before include "Look" , "something to say" .)