Day worker on westbound underground is first-timer (8)
I believe the answer is:
debutant
'first-timer' is the definition.
(I've seen this in another clue)
'day worker on westbound underground' is the wordplay.
'day' becomes 'd' (abbreviation for day).
'worker' becomes 'ant' (worker is a type of ant).
'on' says to put letters next to each other.
'westbound' shows that the letters should be reversed in order (letters written from right to left).
'underground' becomes 'tube' (the London Underground is nicknamed the Tube).
'tube' reversed gives 'ebut'.
'ant' after 'ebut' is 'ebutant'.
'd'+'ebutant'='DEBUTANT'
'is' acts as a 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" .)