One keeping an eye on a companion in the water (7)
I believe the answer is:
watcher
'one keeping an eye' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'companion in the water' is the wordplay.
'companion' becomes 'ch' (Companion of Honour).
'in the' means one lot of letters goes inside another.
'ch' inserted inside 'water' is 'WATCHER'.
'on a' is the link.
Can you help me to learn more?
(Other definitions for watcher that I've seen before include "I observe" , "One who observes" , "Big Brother?" , "Alert observer" , "Onlooker" .)