Record large number used chlorine (9)
I believe the answer is:
chronicle
'record' is the definition.
(I know this)
'large number used chlorine' is the wordplay.
'large number' becomes 'c'.
'used' is an anagram indicator.
'chlorine' with letters rearranged gives 'hronicle'.
'c'+'hronicle'='CHRONICLE'
(Other definitions for chronicle that I've seen before include "Account of important events" , "(Make a) historical record" , "Continuous record of events in order of time" , "(Keep an) historical record" , "History" .)