Disparity in blood group grasped by doctor (3)
I believe the answer is:
gap
'disparity' is the definition.
(gap is a kind of disparity)
'blood group grasped by doctor' is the wordplay.
'blood group' becomes 'a' (A is an example).
'grasped by' is an insertion indicator (I've seen this in other clues).
'doctor' becomes 'gp' (general practitioner**).
'a' placed within 'gp' is 'GAP'.
'in' acts as a link.
(Other definitions for gap that I've seen before include "Narrow opening" , "Aperture" , "Break, in a fence say" , "Break in continuity" , "Open space between two things, mind it" .)