Drink this periodically, not out to lunch (6)
I believe the answer is:
tisane
'drink' is the definition.
(tisane is a kind of drink)
'this periodically not out to lunch' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'not out to lunch' becomes 'sane' (I am not sure about this - if you are sure you should believe this answer much more).
The alternating letters of 'this' are 'ti'.
'ti'+'sane'='TISANE'
Can you help me to learn more?
(Other definitions for tisane that I've seen before include "Herb-tea" , "Herbal infusion" , "Herb tea" , "Infusion (eg of herbs or flowers)" , "Herbal tea drink" .)