Profitable discovery coming from day trip at sea (3,4)
I believe the answer is:
pay dirt
'profitable discovery' is the definition.
(I've seen this in another clue)
'day trip at sea' is the wordplay.
'at sea' indicates anagramming the letters.
'day'+'trip'='daytrip'
'daytrip' is an anagram of 'PAY-DIRT'.
'coming from' acts as a link.
(Other definitions for pay dirt that I've seen before include "Rich ore-bearing ground (US)" , "Ground containing ore in profitable amounts" , "profitable success" .)