Very old priest involved with choir (11)
I believe the answer is:
prehistoric
'very old' is the definition.
(I've seen this before)
'priest involved with choir' is the wordplay.
'involved with' indicates anagramming the letters (involved can mean confusing or complex).
'priest' anagrammed with 'choir' makes 'PREHISTORIC'.
(Other definitions for prehistoric that I've seen before include "Very old or out of date" , "Ancient" , "Primeval" , "very early" , "antiquated" .)