Round tin Drake finally dropped in sea (5)
I believe the answer is:
ocean
'sea' is the definition.
(I know that sea can be written as ocean)
'round tin drake finally dropped in' is the wordplay.
'round' becomes 'o' (round can mean a round object).
'tin' becomes 'can' (synonyms).
'finally' says to take the final letters.
'dropped in' is an insertion indicator.
The last letter of 'drake' is 'e'.
'can' enclosing 'e' is 'cean'.
'o'+'cean'='OCEAN'
(Other definitions for ocean that I've seen before include "Extensive sea area" , "Vast sea" , "A mighty sea" , "lot of liquid" , "Salt water expanse" .)