Fish, mostly worthless, carried by tide on drifting away (3,4)
I believe the answer is:
sea bass
'fish' is the definition.
(sea bass is a kind of fish)
'mostly worthless carried by tide on drifting away' is the wordplay.
'mostly worthless' becomes 'bas' (I can't justify this - if you can you should believe this answer much more).
'carried by' is an insertion indicator.
'tide' becomes 'season'.
'drifting away' indicates named letters should be taken away.
'season' with 'on' removed is 'seas'.
'bas' put within 'seas' is 'SEA-BASS'.
(Other definitions for sea bass that I've seen before include "one showing off plenty of scales" , "Saltwater fish" , "Edible marine fish" , "Marine food fish" .)