A couple of lines on issue raised may be so far unsurpassed (3-4)
I believe the answer is:
all-time
'be so far unsurpassed' is the definition.
(as in an all-time record)
'a couple of lines on issue raised' is the wordplay.
'couple of lines' becomes 'll' (two abbreviations for line).
'on' says to put letters next to each other.
'issue' becomes 'emit' (both can mean to give out or radiate).
'raised' says the letters should be written backwards (going up in a down clue).
'emit' back-to-front is 'time'.
'a'+'ll'+'time'='ALL-TIME'
'may' is the link.
(Other definitions for all-time that I've seen before include "Best ever" , "As yet unsurpassed" , "Hitherto unsurpassed" , "So far unsurpassed" , "Unsurpassed to date" .)