A number outside shelter at first dry (7)
I believe the answer is:
thirsty
'dry' is the definition.
(I've seen this before)
'a number outside shelter at first' is the wordplay.
'a number' becomes 'thirty' (thirty is a kind of number**).
'outside' indicates putting letters inside.
'at first' suggests taking the first letters.
The initial letter of 'shelter' is 's'.
'thirty' going around 's' is 'THIRSTY'.
(Other definitions for thirsty that I've seen before include "Feeling the need to drink" , "Wanting a drink" , "Feeling dehydrated" , "Dry" , "Hungry" .)