Opening article penned by doctor (3)
I believe the answer is:
gap
'opening' is the definition.
(gap is a kind of opening)
'article penned by doctor' is the wordplay.
'article' becomes 'a' (indefinite article).
'penned by' means one lot of letters goes inside another.
'doctor' becomes 'gp' (general practitioner**).
'a' inserted within 'gp' is 'GAP'.
(Other definitions for gap that I've seen before include "Pause" , "prospective students may enjoy this year?" , "Interval, space" , "Open space between two things, mind it" , "Break in continuity" .)