Head of coven enters accompanied by hag (5)
I believe the answer is:
witch
'hag' is the definition.
('witch' can be a synonym of 'hag')
'head of coven enters accompanied by' is the wordplay.
'head of' indicates taking the first letters.
'enters' is an insertion indicator.
'accompanied by' becomes 'with' (I've seen this before).
The initial letter of 'coven' is 'c'.
'c' going into 'with' is 'WITCH'.
(Other definitions for witch that I've seen before include "Black cat and broomstick owner?" , "She has bad spells" , "... and magician" , "Bad speller!" , "Enchantress" .)