Impressive eastern prince seen regularly (4)
I believe the answer is:
epic
'impressive' is the definition.
(epic can mean grand or impressive)
'eastern prince seen regularly' is the wordplay.
'eastern' becomes 'e'.
'seen regularly' indicates alternate letters.
The alternate letters of 'prince' are 'pic'.
'e'+'pic'='EPIC'
(Other definitions for epic that I've seen before include "Larger than life" , "blockbuster" , "long poem telling of heroic deeds" , "(Of) immense (proportions)" , "Film or book on a grand scale" .)