Such action will be involuntary (6)
I believe the answer is:
reflex
'involuntary' is the definition.
(similar in meaning)
'such action' is the wordplay.
'such' becomes 're' (I can't justify this - if you can you should give a lot more credence to this answer).
'action' becomes 'flex' (flex is a kind of action).
're'+'flex'='REFLEX'
'will be' acts as a link.
Can you help me to learn more?
(Other definitions for reflex that I've seen before include "Responsive" , "Bent backwards" , "Automatic instinctive reaction" , "Involuntary (action)" , "Unconscious response to stimulus" .)