Periodically slurp apple brandy finally in stock (6)
I believe the answer is:
supply
'stock' is the definition.
(I've seen this before)
'periodically slurp apple brandy finally' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'finally' says to take the final letters.
The final letter of 'brandy' is 'y'.
'slurp'+'apple'='slurpapple'
The alternate letters of 'slurpapple' are 'suppl'.
'suppl'+'y'='SUPPLY'
'in' acts as a link.
(Other definitions for supply that I've seen before include "Furnish goods or services" , "Lay on" , "in a compliant manner" , "Fund" , "store" .)