They dread becoming dry (9)
I believe the answer is:
dehydrate
'dry' is the definition.
(dehydrating is a kind of drying)
'they dread becoming' is the wordplay.
'becoming' is an anagram indicator.
'they'+'dread'='theydread'
'theydread' with letters rearranged gives 'DEHYDRATE'.
(Other definitions for dehydrate that I've seen before include "Preserve by removing all liquid from" , "Deprive of water" , "Remove water from" , "Remove moisture from, make dry" , "Lose a lot of water" .)