Greek god - son's a Norse god (5)
I believe the answer is:
satyr
'greek god' is the definition.
(satyr is a kind of greek god)
'son's a norse god' is the wordplay.
'norse god' becomes 'tyr' (Tyr is an example).
's'+'a'+'tyr'='SATYR'
(Other definitions for satyr that I've seen before include "Mythical woodland god or grossly lustful man" , "Chaser of wine and women" , "Greek sylvan deity" , "Goat-man attending Bacchus" , "Goatish deity" .)