He's on the qui vive putting the key in (6)
I believe the answer is:
albert
'he's' is the definition.
'the qui vive putting the key in' is the wordplay.
'the qui vive' becomes 'alert' (synonyms).
'putting the key' becomes 'b' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'alert' placed around 'b' is 'ALBERT'.
'on' is the link.
Can you help me to learn more?
(Other definitions for albert that I've seen before include "Prince of Monaco" , "Kind of watch chain" , "Victoria's partner" , "Victim of lion" , "- - Finney" .)