Mountain area — it’s essential to plan descent (5)
I believe the answer is:
andes
'mountain area' is the definition.
(the Andes are a mountain range)
'essential to plan descent' is the wordplay.
'essential to' indicates a hidden word (I've seen this in other clues).
'ANDES' can be found hidden inside 'plan descent'.
'it's' acts as a link.
(Other definitions for andes that I've seen before include "Danes seen in mountains" , "Peak District" , "South American mountains" , "Aconcagua the highest in this range" , "World's longest mountain range" .)