Understanding and looking for missing king (6)
I believe the answer is:
seeing
'understanding' is the definition.
('see' can be a synonym of 'understand')
'looking for missing king' is the wordplay.
'looking for' becomes 'seeking' ('seek' can be a synonym of 'look for').
'missing' is a deletion indicator.
'king' becomes 'k'.
'seeking' with 'k' taken out is 'SEEING'.
'and' is the link.
(Other definitions for seeing that I've seen before include "Observing - understanding" , "Meeting" , "Viewing" , "Watching" , ". . . . . . is believing, they say" .)