New GI I trust - a musician (9)
I believe the answer is:
guitarist
'musician' is the definition.
(guitarist is a kind of musician)
'new gi i trust a' is the wordplay.
'new' indicates anagramming the letters.
'gi'+'i'+'trust'+'a'='giitrusta'
'giitrusta' with letters rearranged gives 'GUITARIST'.
(Other definitions for guitarist that I've seen before include "Player of a fretted instrument" , "Player of a particular stringed instrument" , "Eg, Django Reinhardt" , "Axeman" , "String player" .)