Deserts are sandy (7)
I believe the answer is:
strands
'deserts' is the definition.
(stranding is a kind of deserting)
'are sandy' is the wordplay.
I cannot really understand how this works, but
'are' could be 'ran' (running is a kind of being) and 'ran' is found within the answer.
The remaining letters 'stds' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for strands that I've seen before include "Threads or beaches" , "Leaves helpless on the sand" , "Filaments" , "Maroons" , "Threads, fibres" .)