Oscar is lost at sea, he's all alone (7)
I believe the answer is:
soloist
'he's all alone' is the definition.
'oscar is lost at sea' is the wordplay.
'oscar' becomes 'o' (phonetic alphabet: alpha, bravo, charlie etc.).
'at sea' is an anagram indicator.
'o'+'is'+'lost'='oislost'
'oislost' with letters rearranged gives 'SOLOIST'.
(Other definitions for soloist that I've seen before include "Main singer" , "maybe a vocal type" , "Virtuoso player" , "One who performs" , "musician at front of stage?" .)