Reflex actions essentially spot-on (5)
I believe the answer is:
exact
'spot-on' is the definition.
(I've seen this before)
'reflex actions essentially' is the wordplay.
'essentially' indicates the answer is hidden within the clue (essence can mean the inside of something).
'EXACT' is hidden within 'reflex actions'.
(Other definitions for exact that I've seen before include "Particular" , "Precise and strictly correct" , "Demand payment" , "Explicit" , "Force out" .)