Track particle drunk by the gallon (5,4)
I believe the answer is:
theme song
'track' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'particle drunk by the gallon' is the wordplay.
'particle' becomes 'meson' (meson is a kind of particle).
'drunk by' means one lot of letters goes inside another.
'gallon' becomes 'g' (abbreviation for the unit of liquid).
'the'+'g'='theg'
'meson' placed into 'theg' 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" .)