A style issue that’s been raised previously (2,3,4)
I believe the answer is:
at one time
'previously' is the definition.
(thesaurus)
'a style issue that's been raised' is the wordplay.
'style' becomes 'tone' (tone is a kind of style**).
'issue' becomes 'emit' (I've seen this before).
'that's been raised' is a reversal indicator.
'emit' written backwards gives 'time'.
'a'+'tone'+'time'='AT ONE TIME'
(Other definitions for at one time that I've seen before include "together" , "Formerly" , "in former days" , "Taking place simultaneously" , "in concert" .)