Area of study grey to me sadly (8)
I believe the answer is:
geometry
'area of study' is the definition.
(geometry is a discipline or area of study)
'grey to me sadly' is the wordplay.
'sadly' indicates anagramming the letters.
'grey'+'to'+'me'='greytome'
'greytome' is an anagram of 'GEOMETRY'.
(Other definitions for geometry that I've seen before include "Field of pure mathematics" , "Maths of lines, curves and shapes" , "Grey tome (anag.)" , "Mathematics of points, lines etc." , "Branch of maths - ask Euclid" .)