Penned by a writer — a tome on a Greek Goddess (5)
I believe the answer is:
erato
'greek goddess' is the definition.
(one of the Greek muses)
'penned by a writer a tome' is the wordplay.
'penned by' indicates a hidden word.
'ERATO' is hidden in the letters of 'writer a tome'.
'on a' is the link.
(Other definitions for erato that I've seen before include "Poet's muse" , "verses she inspired" , "poetic patroness" , "Source of artistic inspiration" , "Muse of lyric poetry and mime" .)