Costly to the very end, darling (5)
I believe the answer is:
deary
'darling' is the definition.
('deary' can be a synonym of 'darling')
'costly to the very end' is the wordplay.
'costly' becomes 'dear' ('dear' can be a synonym of 'costly').
'to' is a charade indicator (letters next to each other).
'end' indicates one should take the final letters.
The last letter of 'very' is 'y'.
'dear'+'y'='DEARY'
(Other definitions for deary that I've seen before include "Ready (anag) -- pet" , "Love" .)