Greek god posed with lyre, undressed (5)
I believe the answer is:
satyr
'greek god' is the definition.
(satyr is a kind of greek god)
'posed with lyre undressed' is the wordplay.
'posed' becomes 'sat' (posed or sat for an artist).
'with' is a charade indicator (letters next to each other).
'undressed' indicates the central letters (the word removes the letters which clothe it).
The middle letters of 'lyre' are 'yr'.
'sat'+'yr'='SATYR'
(Other definitions for satyr that I've seen before include "lecher" , "Trays (anag.)" , "Lecherous woodland goat-man deity" , "Goatish deity" , "Chaser of wine and women" .)