Respond with about turn (5)
I believe the answer is:
react
'respond' is the definition.
(to react to something is to respond to it)
'with about turn' is the wordplay.
'with' is a charade indicator (letters next to each other).
'about' becomes 're' ('re' means regarding or about).
'turn' becomes 'act' (theatre).
're' next to 'act' is 'REACT'.
(Other definitions for react that I've seen before include "Respond to a stimulus" , "Act as opponent" , "Change chemically" , "What a chemical might do" , "Acknowledge" .)