Watches every second of expert (3)
I believe the answer is:
ace
'expert' is the definition.
(ace can informally mean very good)
'watches every second of' is the wordplay.
'every second of' indicates alternate letters.
The alternating letters of 'watches' are 'ACE'.
(Other definitions for ace that I've seen before include "Crack" , "Expert; one" , "winner in court" , "hotshot" , "one among a set of diamonds" .)