Cautious manner restrains king (4)
I believe the answer is:
wary
'cautious' is the definition.
('wary' can be similar in meaning to 'cautious')
'manner restrains king' is the wordplay.
'manner' becomes 'way' ('way' can be a synonym of 'manner').
'restrains' indicates putting letters inside.
'king' becomes 'r' (r is abbreviation for rex).
'way' placed around 'r' is 'WARY'.
(Other definitions for wary that I've seen before include "Displaying caution" , "Cautious, suspicious" , "Cautious - vigilant" , "Circumspect" , "Watchful" .)