Sleuth in a deli regularly knocked back drink (3,3)
I believe the answer is:
ice tea
'drink' is the definition.
(ice tea is a kind of drink)
'sleuth in a deli regularly knocked back' is the wordplay.
'sleuth' becomes 'tec' (short for detective).
'in' indicates putting letters inside.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'knocked back' says the letters should be written in reverse.
The alternate letters of 'deli' are 'ei'.
'a'+'ei'='aei'
'tec' put into 'aei' is 'aeteci'.
'aeteci' backwards is 'ICE TEA'.
(Other definitions for ice tea that I've seen before include "refresher" , "Cold summer drink" , "Cool non-alcoholic drink" , "Chilled drink flavoured with lemon" , "refreshing drink" .)