Day worker on westbound underground is first-timer (8)
I believe the answer is:
debutant
'first-timer' is the definition.
(I've seen this before)
'day worker on westbound underground' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'worker' becomes 'ant' (worker is a type of ant).
'on' is a charade indicator (letters next to each other).
'westbound' is a reversal indicator (letters written from right to left).
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'tube' back-to-front is 'ebut'.
'ant' after 'ebut' is 'ebutant'.
'd'+'ebutant'='DEBUTANT'
'is' is the link.
(Other definitions for debutant that I've seen before include "Someone making their first appearance" , "One making first appearance" , "Newcomer to team" , "Beginner, first-timer" .)