A bird shedding hard patch (4)
I believe the answer is:
area
'a' is the definition.
(maths abbreviation)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for area that I've seen before include "field of study" , "Space occupied by something" , "Patch" , "part of town" , "Space for a specific purpose, say picnic" .)