Wild sandy shores lose initial hydration when it doesn't rain (3,7)

'wild sandy shores' is the definition.
Although both the answer and definition are plural nouns, I can't see how they can define each other.

'lose initial hydration when it doesn't rain' is the wordplay.
I cannot really see how this works, but
'when' could be 'on' (I've seen this in other clues) and 'on' is found within the answer.
'it' could be 'e' ('e' can mean 'electronic' which is similar to 'IT') and 'e' is found in the answer.
This explanation may well be incorrect...

Can you help me to learn more?