Doctor penning article: editor looked astonished (5)
I believe the answer is:
gaped
'looked astonished' is the definition.
(to gape is to stare at in shock)
'doctor penning article editor' is the wordplay.
'doctor' becomes 'gp' (general practitioner).
'penning' is an insertion indicator.
'article' becomes 'a' (the indefinite article in English).
'editor' becomes 'ed'.
'gp' going around 'a' is 'gap'.
'gap'+'ed'='GAPED'
(Other definitions for gaped that I've seen before include "Opened mouth wide" , "shown astonishment" , "Was yawning" , "Looked stunned" , "Stared open-mouthed - yawned" .)