A number eating seconds dehydrated (7)
I believe the answer is:
thirsty
'dehydrated' is the definition.
('thirsty' can be similar in meaning to 'dehydrated')
'a number eating seconds' is the wordplay.
'a number' becomes 'thirty' (thirty is a kind of number**).
'eating' indicates putting letters inside.
'seconds' becomes 's'.
'thirty' placed around 's' is 'THIRSTY'.
(Other definitions for thirsty that I've seen before include "Parched" , "Needing moisture" , "Wanting a drink" , "Ardently desiring" , "Gasping" .)