A player - one in new boots (6)
I believe the answer is:
oboist
'a player' is the definition.
(oboe player)
'one in new boots' is the wordplay.
'one' becomes 'i' (Roman numeral).
'in' indicates putting letters inside.
'new' indicates an anagram.
'boots' anagrammed gives 'obost'.
'i' going within 'obost' is 'OBOIST'.
(Other definitions for oboist that I've seen before include "orchestra member" , "Instrumentalist" , "Do I boost the musician?" , "Player of woodwind instrument of treble pitch" , "Wind player" .)