Turned to display drowned valley (3)
I believe the answer is:
ria
'drowned valley' is the definition.
(I've seen this before)
'turned to display' is the wordplay.
'turned' says the letters should be written in reverse.
'to display' becomes 'air' (I've seen this before).
'air' reversed gives 'RIA'.
(Other definitions for ria that I've seen before include "inlet between hills" , "Institution for the promotion of art, in short" , "Coastal inlet" , "Little girl" , "Flooded valley" .)