Failing to conceal bottom, one’s skimpy costume (7)
I believe the answer is:
tankini
'failing to conceal bottom' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'one's skimpy costume' is the wordplay.
I cannot really see how this works, but
'one' could be 'an' (an apple is one apple) and 'an' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for tankini that I've seen is " gear".)