An identifying melody on G follows the music’s first two directions (5,4)
I believe the answer is:
theme song
'an identifying melody' is the definition.
'theme song' can be an answer for 'melody' (theme song is a kind of melody). I am unsure of the 'an identifying' bit.
'on g follows the music's first two directions' is the wordplay.
'two directions' can mean two replacements for 'direction'.
'follows' is a charade indicator (letters next to each other).
'first' suggests taking the first letters.
'direction' becomes 'e' (E is an example).
'direction' becomes 's' (S is an example).
The initial letter of 'music' is 'm'.
'on'+'g'='ong'
'the'+'m'+'e'+'s'='themes'
'ong' after 'themes' is 'THEME SONG'.
Can you help me to learn more?
(Other definitions for theme song that I've seen before include "Characteristic melody often repeated in musical" , "Signature tune" , "Give me Sunshine" .)