Annoy Head of English after in-depth talk occasionally (6)
I believe the answer is:
nettle
'annoy' is the definition.
(I know that annoy can be written as nettle)
'head of english after in-depth talk occasionally' is the wordplay.
'head of' suggests taking the first letters.
'after' is a charade indicator (letters next to each other).
'occasionally' means one should take alternating letters.
The initial letter of 'english' is 'e'.
'in'+'depth'+'talk'='indepthtalk'
The alternate letters of 'indepthtalk' are 'nettl'.
'e' put after 'nettl' is 'NETTLE'.
(Other definitions for nettle that I've seen before include "anger?" , "Upset" , "Growing" , "Provoke" , "Irritate - weed" .)