Timekeepers extremely eager, but not this one (12)
I believe the answer is:
clockwatcher
'timekeepers extremely eager' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'not this one' is the wordplay.
I cannot really understand how this works, but
'not' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
This explanation may well be incorrect...
'but' is the link.
Can you help me to learn more?
(Another definition for clockwatcher that I've seen is " One eager to leave bang on time".)