Two-faced male doctor has external face (8)
I believe the answer is:
dihedral
'two-faced' is the definition.
(I've seen this before)
'male doctor has external face' is the wordplay.
'male' becomes 'he'.
'doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'has external' is an insertion indicator.
'face' becomes 'dial' (dial is a kind of face**).
'he'+'dr'='hedr'
'hedr' inserted within 'dial' is 'DIHEDRAL'.
(Other definitions for dihedral that I've seen before include "Two-faced" , "angle" , "Upward inclination of a plane's wings" , "surrounded by a couple of planes" .)