Spirit is a trademark, at the end of the day (6)
I believe the answer is:
brandy
'spirit' is the definition.
(alcoholic spirit)
'trademark at the end of the day' is the wordplay.
'trademark' becomes 'brand' (I've seen this before).
'at the end of' indicates one should take the final letters.
The final letter of 'day' is 'y'.
'brand'+'y'='BRANDY'
'is a' is the link.
(Other definitions for brandy that I've seen before include "Distilled alcohol" , "warmer?" , "Strong spirit distilled from wine" , "Eau de vie" , "Marc, say" .)