Boxer, once lethargic, finally is on song (4)
I believe the answer is:
clay
'boxer once' is the definition.
(Muhammad Ali was born Cassius Clay)
'lethargic finally is on song' is the wordplay.
'finally' indicates one should take the final letters.
'is on' is a charade indicator (letters next to each other).
'song' becomes 'lay' (lay is a kind of song**).
The final letter of 'lethargic' is 'c'.
'c'+'lay'='CLAY'
(Other definitions for clay that I've seen before include "Moulding material" , "Court surface" , "Material for idol's feet?" , "Heavy soil" , "Brick/pottery substance" .)