Departs too soon, at significant cost (6)
I believe the answer is:
dearly
'at significant cost' is the definition.
I can't judge whether this defines the answer.
'departs too soon' is the wordplay.
'departs' becomes 'd' (common abbreviation e.g. train timetables).
'too soon' becomes 'early' (earlier than required).
'd'+'early'='DEARLY'
Can you help me to learn more?
(Other definitions for dearly that I've seen before include "At a high price - with fondness" , "with much suffering" , "Extremely - devotedly - at great cost" , "At high cost" , "Very much; at great cost" .)