One may have kept a watch on him (6)
I believe the answer is:
albert
'one may have' is the definition.
The definition suggests a singular noun which matches the answer.
'a watch on him' is the wordplay.
'a watch on' becomes 'al' (I can't explain this - if you can you should believe this answer much more).
'him' becomes 'Bert'.
'al'+'bert'='ALBERT'
'kept' acts as a link.
Can you help me to learn more?
(Other definitions for albert that I've seen before include "Victoria's partner" , "Victim of lion" , "Prince of Monaco" , "Queen Victoria's consort or familiar ex-Taoiseach" , "- Square, Walford (EastEnders)" .)