Keeps under cover when it's hot outside (5)
I believe the answer is:
'keeps under cover' is the definition.
Although both the answer and definition are verbs in their -s form, I cannot understand how they can define each other.
'it's hot outside' is the wordplay.
'it's hot' becomes 'h' (abbreviation eg on taps).
'outside' becomes 'ides' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'when' acts as a link.
Can you help me to learn more?
(Other definitions for hides that I've seen before include "lies low", "Dressed skins of animals", "coats", "screens", "Cloaks".)