Area that's dry up tree (3)
I believe the answer is:
ash
'tree' is the definition.
(ash tree is a kind of tree)
'area that's dry up' is the wordplay.
'area' becomes 'a' (maths abbreviation).
'that's dry up' becomes 'sh' (I can't justify this - if you can you should give a lot more credence to this answer).
'a'+'sh'='ASH'
Can you help me to learn more?
(Other definitions for ash that I've seen before include "Fire's remains" , "Burnt remains; tree" , "Result of burning - deciduous tree" , "blonde" , "Woodland tree; result of burning" .)