Red borders in study (7)
I believe the answer is:
crimson
'red' is the definition.
(I know that red can be written as crimson)
'borders in study' is the wordplay.
'borders' becomes 'rims' (rim can mean an edge or border).
'in' indicates putting letters inside.
'study' becomes 'con' (con can informally mean to study).
'rims' going inside 'con' is 'CRIMSON'.
(Other definitions for crimson that I've seen before include "Deep red colour" , "Flush" , "Ruddy" , "Microns of colour" , "Colour of deep blushing?" .)