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 see 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' is the 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" .)