Watches, say, adding seconds (8)
I believe the answer is:
observes
'watches' is the definition.
(I know this)
'say adding seconds' is the wordplay.
'say' becomes 'observe' (observing is a kind of saying).
'adding seconds' becomes 's' (I am not sure about this - if you are sure you should believe this answer much more).
'observe'+'s'='OBSERVES'
Can you help me to learn more?
(Other definitions for observes that I've seen before include "Notices or becomes conscious of" , "Acts as member of royal corps" , "Notices; comments" , "notes" .)