One for the first time playing tuba in pit (8)
I believe the answer is:
debutant
'one for the first time' is the definition.
(someone making a debut appearance)
'playing tuba in pit' is the wordplay.
'playing' is an anagram indicator.
'in' means one lot of letters goes inside another.
'pit' becomes 'dent' (pit can mean a dent or depression).
'tuba' is an anagram of 'buta'.
'buta' going into 'dent' is 'DEBUTANT'.
(Other definitions for debutant that I've seen before include "One making first appearance" , "Beginner, first-timer" , "Newcomer to team" , "Someone making their first appearance" .)