Yours truly led astray by unknown mixture (6)
I believe the answer is:
medley
'mixture' is the definition.
(I've seen this before)
'yours truly led astray by unknown' is the wordplay.
'yours truly' becomes 'me' (the person that set this crossword).
'astray' is an anagram indicator.
'by' says to put letters next to each other.
'unknown' becomes 'y' (algebra).
'led' anagrammed gives 'dle'.
'me'+'dle'+'y'='MEDLEY'
(Other definitions for medley that I've seen before include "A linked group of songs or tunes" , "Various tunes combined in one piece" , "Mixture of tunes etc" , "Piece of music comprising various tunes" , "Group of songs played as one piece" .)