For a start, many take the tube back (5)
I believe the answer is:
debut
'for a start' is the definition.
(debut is a kind of start)
'many take the tube back' is the wordplay.
'many' becomes 'D' (Roman numeral for 500).
'take' is a charade indicator (letters next to each other).
'back' says the letters should be written in reverse.
'tube' reversed gives 'ebut'.
'd'+'ebut'='DEBUT'
(Other definitions for debut that I've seen before include "Appear in public for the first time" , "First appearance, a coming-out" , "Tubed (anag.)" , "The coming out of a society girl" , "First appearance, coming out" .)