Must dash, ta-ta - about to go (5)
I believe the answer is:
gotta
'must' is the definition.
(I've seen this in another clue)
'dash ta-ta about to go' is the wordplay.
'dash' says to take the initial letters (I've seen 'dash of' mean this (a small amount of the word)).
'about' indicates putting letters inside.
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
The initial letter of 'ta' is 't'.
't' going into 'ta' is 'tta'.
'tta' put after 'go' is 'GOTTA'.
(Another definition for gotta that I've seen is " American must see".)