After downpour initially, day three surprisingly dry (9)
I believe the answer is:
dehydrate
'dry' is the definition.
(dehydrating is a kind of drying)
'after downpour initially day three surprisingly' is the wordplay.
'after' says to put letters next to each other.
'initially' indicates taking the first letters.
'surprisingly' is an anagram indicator.
The first letter of 'downpour' is 'd'.
'day'+'three'='daythree'
'daythree' is an anagram of 'ehydrate'.
'd' next to 'ehydrate' is 'DEHYDRATE'.
(Other definitions for dehydrate that I've seen before include "Remove water from" , "Drain" , "Lose a lot of water" , "Preserve by removing all liquid from" , "Deprive of water" .)