They fly although they are powerless to do so (7)
I believe the answer is:
gliders
'they' is the definition.
Both the answer and definition are plural nouns.
Perhaps there's an association between them I don't understand?
'fly although they are powerless to do so' is the wordplay.
I cannot quite see how this works, but
'fly' could be 'glide' (gliding is a kind of flying) and 'glide' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for gliders that I've seen is " Flying vehicles without engines".)