Certainly not retreating, hit out (2,6)
I believe the answer is:
on strike
'out' is the definition.
(I've seen this before)
'certainly not retreating hit' is the wordplay.
'certainly not' becomes 'no'.
'retreating' says the letters should be written backwards.
'hit' becomes 'strike' (synonyms).
'no' backwards is 'on'.
'on'+'strike'='ON STRIKE'
(Other definitions for on strike that I've seen before include "Refusing to work" , "Facing a ball" , "Involved in industrial action" , "Taking industrial action" .)