Wandering minstrel with a tune, mostly musical piece (12)
I believe the answer is:
instrumental
'musical piece' is the definition.
'wandering minstrel with a tune mostly' is the wordplay.
'wandering' indicates anagramming the letters.
'with' means one lot of letters go next to another.
'mostly' means to remove the last letter.
'a'+'tune'='atune'
'atune' with its last letter taken away is 'atun'.
'minstrel'+'atun'='minstrelatun'
'minstrelatun' anagrammed gives 'INSTRUMENTAL'.
(Other definitions for instrumental that I've seen before include "Acting as a means; piece of music without vocals" , "Piece of music without voices" , "Like a musical piece - it's of help" , "case" , "Important" .)