Knocking back eggnog at nephew's, reveals name of place used by foreign army (8)
I believe the answer is:
pentagon
'name of place used by foreign army' is the definition.
I can't tell whether this definition defines the answer.
'knocking back eggnog at nephew's reveals' is the wordplay.
'knocking back' says the letters should be written in reverse.
'reveals' says the answer is hidden in the clue.
'nogatnep' is hidden in the letters of 'eggnog at nephews'.
'nogatnep' written backwards gives 'PENTAGON'.
Can you help me to learn more?
(Other definitions for pentagon that I've seen before include "Five-sided shape" , "military installation" , "Location of the US Department of Defense" , "Five-sided polygon" , "Geometric figure with five sides" .)