'Breathers' are good for what may ail you!
I believe the answer is:
gills
'breathers' is the definition.
Although both the answer and definition are plural nouns, I don't understand how one could define the other.
'good for what may ail you' is the wordplay.
I cannot quite see how this works, but
'good' could be 'g' (abbreviation) and 'g' is present in the answer.
The remaining letters 'ills' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
'are' is the link.
Can you help me to learn more?
(Other definitions for gills that I've seen before include "Fish breathe through them" , "Quantities of drink" .)