Pure daughter leaving disco, it's said (9)
I believe the answer is:
utterance
'it's said' is the definition.
(an utterance is something which is said)
'pure daughter leaving disco' is the wordplay.
'pure' becomes 'utter' (meaning complete or absolute).
'daughter' becomes 'd' (genealogical abbreviation).
'leaving' indicates named letters should be taken away.
'disco' becomes 'dance' (I've seen this in other clues).
'dance' with 'd' removed is 'ance'.
'utter'+'ance'='UTTERANCE'
(Other definitions for utterance that I've seen before include "Vocal expression" , "Word" , "Something said" , "Statement" , "Remark" .)