Round tin Popeye finally dropped in sea (5)
I believe the answer is:
ocean
'sea' is the definition.
('ocean' can be a synonym of 'sea')
'round tin popeye finally dropped in' is the wordplay.
'round' becomes 'o' (round can mean a round object**).
'tin' becomes 'can' ('can' can be a synonym of 'tin').
'finally' says to take the final letters.
'dropped in' is an insertion indicator.
The final letter of 'popeye' is 'e'.
'can' enclosing 'e' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "A lot of water!" , "lot of liquid" , "Quantity of water" , "Indian perhaps" , "Immense expanse or quantity" .)