Player whose buckled boots are out of line (6)
I believe the answer is:
oboist
'player' is the definition.
(oboe player)
'buckled boots are out of line' is the wordplay.
'buckled' indicates anagramming the letters.
'are out' indicates putting letters inside.
'of line' becomes 'i' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'boots' is an anagram of 'obost'.
'obost' going around 'i' is 'OBOIST'.
'whose' acts as a link.
Can you help me to learn more?
(Other definitions for oboist that I've seen before include "Player of woodwind instrument of treble pitch" , "Musician who plays woodwind of treble pitch" , "orchestra member" , "Do I boost the musician?" , "Instrumentalist" .)