One flyer in drink, another being on rescue vessel (10)
I believe the answer is:
meadowlark
'one flyer' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'drink another being on rescue vessel' is the wordplay.
I cannot quite see how this works, but
'drink' could be 'mead' (mead is a kind of drink) and 'mead' is found in the answer.
'on' could be 'o' (I've seen this in other clues) and 'o' is located in the answer.
'vessel' could be 'ark' (ark is a kind of vessel) and 'ark' is found within the answer.
This explanation may well be incorrect...
'in' is the link.
(Other definitions for meadowlark that I've seen before include "Bird" , "Songbird" .)