Turn up ventilation inlet (3)
I believe the answer is:
ria
'inlet' is the definition.
(I've seen this before)
'turn up ventilation' is the wordplay.
'turn up' shows that the letters should be reversed in order.
'ventilation' becomes 'air'.
'air' written backwards gives 'RIA'.
(Other definitions for ria that I've seen before include "Long narrow inlet" , "Institution for the promotion of art, in short" , "Drowned river valley now an inlet of the sea" , "inlet between hills" , "Submerged valley" .)