Taxable ones may be capital (5)
I believe the answer is:
gains
'taxable ones' is the definition.
Although both the answer and definition are plural nouns, I can't see how they can define each other.
'be capital' is the wordplay.
I cannot really understand how this works, but
'capital' could be 'ai' (resembles A1, which can mean capital or excellent) and 'ai' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
'may' is the link.
Can you help me to learn more?
(Other definitions for gains that I've seen before include "Wins" , "Profits" , "Narrows the gap" .)