A month without light infantry attending city shop (12)
I believe the answer is:
delicatessen
'shop' is the definition.
(delicatessen is a kind of shop)
'a month without light infantry attending city' is the wordplay.
'a month' becomes 'dec' (abbreviation for December).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'light infantry' becomes 'li' (abbreviation).
'attending' becomes 'at' (e.g. at school means attending school).
'city' becomes 'essen' (Essen is an example).
'dec' placed around 'li' is 'delic'.
'delic'+'at'+'essen'='DELICATESSEN'
(Other definitions for delicatessen that I've seen before include "Shop selling unusual or foreign prepared food" , "Unusual foods here" , "Rich food had here, perhaps?" , "Rare food shop" , "Fine foods shop" .)