Rain oddly needs a drowned valley (3)
I believe the answer is:
ria
'drowned valley' is the definition.
(I've seen this before)
'rain oddly needs a' is the wordplay.
'oddly' indicates alternate letters.
'needs' says to put letters next to each other.
The alternate letters of 'rain' are 'ri'.
'ri'+'a'='RIA'
(Other definitions for ria that I've seen before include "Coastal inlet" , "Flooded valley" , "inlet between hills" , "Institution for the promotion of art, in short" , "Drowned coastal valley" .)