Area that's dry up tree (3)
I believe the answer is:
Here is my best explanation:
'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).
(Other definitions for ash that I've seen before include "End of fire", "at village near Whitchurch in Shropshire", "blonde", "What's often left after something 1 Ac", "Remnant from fire", "Part of forest", "sort of tree?", "Tree in danger", "Fire remains" and "Tree; burned residue".)