Cut off -- one thus gets behind schedule (7)
I believe the answer is:
isolate
'cut off' is the definition.
(thesaurus)
'one thus gets behind schedule' is the wordplay.
'one' becomes 'i' (Roman numeral).
'thus' becomes 'so' ('so' can be a synonym of 'thus').
'gets' means one lot of letters go next to another.
'behind schedule' becomes 'late' (I've seen this before).
'i'+'so'+'late'='ISOLATE'
(Other definitions for isolate that I've seen before include "split up" , "Place apart from others or alone" , "Cut off; quarantine" , "Set apart from others" , "Separate and keep away from other people" .)