Flyer ruinous to cave (6)
I believe the answer is:
avocet
'flyer' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'ruinous to cave' is the wordplay.
'ruinous' indicates anagramming the letters.
'to'+'cave'='tocave'
'tocave' with letters rearranged gives 'AVOCET'.
Can you help me to learn more?
(Other definitions for avocet that I've seen before include "Shorebird - octave (anag)" , "RSPB emblem" , "Curved-bill shore bird" , "Wading bird with long bill" , "Wading bird - octave (anag)" .)