I'm in the sea, but can manage without! (5)
I believe the answer is:
waive
'the sea but can manage without' is the definition.
I can't judge whether this defines the answer.
'i'm' is the wordplay.
I cannot quite understand how this works, but
'i'm' could be 'i' (I've seen this in other clues) and 'i' is found within the answer.
The remaining letters 'wave' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in' acts as a link.
Can you help me to learn more?
(Other definitions for waive that I've seen before include "Give up" , "pass up" , "Dispense with, forgo" , "Not insist on (claim)" , "Refrain from insisting on rights" .)