Greek in underworld, Leander seen regularly (7)
I believe the answer is:
hellene
'greek' is the definition.
(term for an ancient Greek)
'underworld leander seen regularly' is the wordplay.
'underworld' becomes 'Hell' (synonyms).
'seen regularly' indicates alternate letters.
The alternating letters of 'leander' are 'ene'.
'hell'+'ene'='HELLENE'
'in' is the link.
(Other definitions for hellene that I've seen before include "old Greek" , "Greek girl" .)