Complains on GMB about old ecclesiastical music (9)
I believe the answer is:
plainsong
'old ecclesiastical music' is the definition.
'plainsong' can be an answer for 'music' (plainsong is a kind of music). I'm not certain of the 'old ecclesiastical' bit.
'complains on gmb about' is the wordplay.
'about' indicates the answer is hidden within the clue (the clue goes around or about the answer).
'PLAINSONG' is hidden in the letters of 'complains on gmb'.
Can you help me to learn more?
(Other definitions for plainsong that I've seen before include "Mediaeval church music" , "Simple melody" , "church melody" , "Lied, perhaps" , "Medieval style of church music" .)