Must be off - heading to town - cheers! (5)

I believe the answer is:

gotta

'must' is the definition.
(I've seen this before)

'be off heading to town cheers' is the wordplay.
'be off' becomes 'go'.
'heading to' says to take the initial letters (the head of the word).
'cheers' becomes 'ta' (both can mean thank you).
The initial letter of 'town' is 't'.
'go'+'t'+'ta'='GOTTA'

(Another definition for gotta that I've seen is " American must see".)

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