Sea water in one's watch (10)
I believe the answer is:
mainspring
'one's watch' is the definition.
The definition and answer can be both man-made objects as well as being singular nouns.
Perhaps they are linked in a way I don't understand?
'sea water' is the wordplay.
'sea' becomes 'main' (synonyms).
'water' becomes 'spring' (spring water is a kind of water).
'main'+'spring'='MAINSPRING'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for mainspring that I've seen before include "Principal agent of motivation" , "Pining rams (anag.)" , "motivating force" , "It motivates" , "essential to watch" .)