Breathing aids, no less, when swimming around outside of rock (8)
I believe the answer is:
snorkels
'breathing aids' is the definition.
Although both the answer and definition are verbs in their -s form, I don't see how one could define the other.
'no less when swimming around outside of rock' is the wordplay.
'when swimming' is an anagram indicator.
'around' means one lot of letters goes inside another.
'outside of rock' becomes 'rk' (I can't justify this - if you can you should give a lot more credence to this answer).
'no'+'less'='noless'
'noless' with letters rearranged gives 'snoels'.
'snoels' going around 'rk' is 'SNORKELS'.
Can you help me to learn more?
(Another definition for snorkels that I've seen is " Breathing devices for swimmers".)