Red borders in study (7)
I believe the answer is:
crimson
'red' is the definition.
(I know that crimson is a type of red)
'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 mean to study).
'rims' put into '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?" .)