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Track particle drunk by the gallon (5,4)

Ross

I believe the answer is:

theme song

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'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" .)

I've seen this clue in The Telegraph.
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