One extremely tardy in divorce (7)
I believe the answer is:
isolate
'divorce' is the definition.
(I've seen this before)
'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'
'in' acts as a link.
(Other definitions for isolate that I've seen before include "Maroon" , "Place apart, away from others" , "Pick out" , "Seclude" , "put away?" .)