... set apart from one extremely tardy (7)
I believe the answer is:
isolate
'set apart' is the definition.
(synonyms)
'one extremely tardy' is the wordplay.
'one' becomes 'I' (Roman numeral).
'extremely' becomes 'so' ('so' can mean 'extremely' or 'very').
'tardy' becomes 'late' (synonyms).
'i'+'so'+'late'='ISOLATE'
'from' acts as a link.
(Other definitions for isolate that I've seen before include "Segregate or quarantine" , "Point out" , "divorce" , "Set apart, quarantine" , "Pick out" .)