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' (physics symbol).
'going back' says the letters should be written backwards.
'no' in reverse letter order is 'on'.
'express'+'i'+'on'='EXPRESSION'
'of' acts as a link.
(Other definitions for expression that I've seen before include "Look" , "something to say" .)