500 underground coming up for a start (5)
I believe the answer is:
debut
'a start' is the definition.
(debut is a kind of start)
'500 underground coming up' is the wordplay.
'500' becomes 'd' (Roman numeral for 500).
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'coming up' says the letters should be written backwards.
'tube' backwards is 'ebut'.
'd'+'ebut'='DEBUT'
'for' is the link.
(Other definitions for debut that I've seen before include "entrance" , "First appearance, coming out" , "Initial appearance" , "opening" , "The coming out of a society girl" .)