Mend or replace so that it's up-to-date (6)
I believe the answer is:
modern
'that it's up-to-date' is the definition.
(I've seen this before)
'mend or replace' is the wordplay.
'replace' is an anagram indicator (I've seen 'replacing' mean this (placing again)).
'mend'+'or'='mendor'
'mendor' is an anagram of 'MODERN'.
'so' acts as a link.
(Other definitions for modern that I've seen before include "'Contemporary, up-to-date (6)'" , "Of the present time or current fashion" , "'Contemporary, up-to-the-minute (6)'" , "In history, the period or era since the Middle Ages" , "The very latest available" .)