Departs before time and at great cost (6)
I believe the answer is:
dearly
'at great cost' is the definition.
(synonyms)
'departs before time' is the wordplay.
'departs' becomes 'd' (abbreviation eg in train timetables).
'before time' becomes 'early' (I've seen this before).
'd'+'early'='DEARLY'
'and' acts as a link.
(Other definitions for dearly that I've seen before include "At high cost" , "Very much; at great cost" , "paying for big mistakes like this!" , "Extremely - devotedly - at great cost" , "with much suffering" .)