King holds back magical creature — it's instinctive (6)
I believe the answer is:
reflex
'instinctive' is the definition.
(I've seen this before)
'king holds back magical creature' is the wordplay.
'king' becomes 'rex' ('king' in Latin).
'holds' means one lot of letters goes inside another.
'back' shows that the letters should be reversed in order.
'magical creature' becomes 'elf' (creature in fairy tales).
'elf' written backwards gives 'fle'.
'rex' placed around 'fle' is 'REFLEX'.
'it's' is the link.
(Other definitions for reflex that I've seen before include "Movement in response to a stimulus" , "Sneeze, blink or hiccup?" , "Action not requiring conscious thought" , "Involuntary (action)" , "Knee-jerk" .)