Fine whiskey found in barrel back in the day (10)
I believe the answer is:
yesteryear
'back in the day' is the definition.
(yesteryear means in the past)
'fine whiskey found in barrel' is the wordplay.
'fine' becomes 'yes' ('fine' is a way of saying 'yes').
'whiskey' becomes 'rye whiskey' (rye whiskey is a type of whiskey).
'found in' is an insertion indicator.
'barrel' becomes 'tear' (both can mean to move fast).
'rye' placed into 'tear' is 'teryear'.
'yes'+'teryear'='YESTERYEAR'
(Other definitions for yesteryear that I've seen before include "Days gone by" , "nostalgic time, perhaps" , "Time disappeared" , "Rossetti's translation questioned whereabouts of its snows!" , "Former times" .)