Responsive to stimulus concerning voice (8)
I believe the answer is:
reactive
'responsive to stimulus' is the definition.
(I've seen this before)
'concerning voice' is the wordplay.
'concerning' becomes 're' (both can mean on a particular topic).
'voice' becomes 'active' (active voice is a kind of voice).
're'+'active'='REACTIVE'
(Other definitions for reactive that I've seen before include "Producing responses" , "Showing a response to stimuli" , "Responding to stimuli" , "Responsive to stimulus" , "Responding to stimulus" .)