Deep source of power, perhaps (4)
I believe the answer is:
main
I believe this is a double definition.
'deep' is the first definition.
(both can refer to the sea)
'source of power perhaps' is the second definition.
(eg electrical mains)
(Other definitions for main that I've seen before include "Chief - principal electricity cable" , "Principal motivation" , "Source of water" , "Dominant" , "Chief in importance" .)