Dignitary occasionally interrupted by discourteous gatecrasher (8)
I believe the answer is:
intruder
'gatecrasher' is the definition.
(I've seen this before)
'dignitary occasionally interrupted by discourteous' is the wordplay.
'occasionally' indicates alternate letters (letters taken at regular intervals or occasions).
'interrupted by' indicates putting letters inside.
'discourteous' becomes 'rude' (I've seen this before).
The alternate letters of 'dignitary' are 'intr'.
'intr' enclosing 'rude' is 'INTRUDER'.
(Other definitions for intruder that I've seen before include "One entering uninvited" , "Unwanted visitor" , "Gatecrasher" , "Person who comes in being neither invited nor wanted" , "Trespasser" .)