Cut off one so tardy (7)
I believe the answer is:
isolate
'cut off' is the definition.
(thesaurus)
'one so tardy' is the wordplay.
'one' becomes 'i' (Roman numeral).
'tardy' becomes 'late' ('late' can be a synonym of 'tardy').
'i'+'so'+'late'='ISOLATE'
(Other definitions for isolate that I've seen before include "Segregate or quarantine" , "Place or set apart" , "Cut off from all contact" , "Also tie (anag.)" , "Place in seclusion" .)