Hard to follow old scent (5)
I believe the answer is:
odour
'scent' is the definition.
(synonyms)
'hard to follow old' is the wordplay.
'hard' becomes 'dour' (I've seen this in other clues).
'to follow' is a charade indicator (letters next to each other).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'dour' after 'o' is 'ODOUR'.
(Other definitions for odour that I've seen before include "Bouquet" , "Perfume" , "Any property detected by the olfactory system" , "Regard" , "essence" .)