Two sides missing from Lewis's song (5)
I believe the answer is:
carol
'song' is the definition.
(I know that carol is a type of song)
'two sides missing from lewis's' is the wordplay.
'two sides' becomes 'LR' (abbreviations for left and right).
'missing from' suggests deleting specific letters.
'lewis' becomes 'carroll' (I've seen this before).
'carroll' with 'lr' removed is 'CAROL'.
(Other definitions for carol that I've seen before include "Religious song - girl's name" , "Joyful religious song" , "sing happily" , "Christmas ditty" , "Canticle" .)