Solitary marine creature abandoned by sailor (5)
I believe the answer is:
alone
'solitary' is the definition.
(synonyms)
'marine creature abandoned by sailor' is the wordplay.
'marine creature' becomes 'abalone' (I've seen this before).
'abandoned by' suggests deleting specific letters.
'sailor' becomes 'ab' (abbreviation for able seaman).
'abalone' with 'ab' taken out is 'ALONE'.
(Other definitions for alone that I've seen before include "Without friends" , "Solo" , "Exclusively, by oneself" , "without partner?" , "Isolated; exclusively" .)