One might catch you saying it doesn’t measure involuntary responses (9)
I believe the answer is:
polygraph
'one might catch' is the definition.
The definition suggests a singular noun which matches the answer.
'saying it doesn't measure involuntary responses' is the wordplay.
I cannot really understand how this works, but
'measure' could be 'ph' (pH is a kind of measure) and 'ph' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'you' acts as a link.
Can you help me to learn more?
(Other definitions for polygraph that I've seen before include "Lie-detector machine" , "Machine used as lie detector" , "Shower of pork pies" , "Electronic instrument used as a lie detector" .)