Have quick look in either direction (4)
I believe the answer is:
peep
'have quick look' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'in either direction' is the wordplay.
'in' is an insertion indicator.
'either' becomes 'pep' (I can't justify this - if you can you should believe this answer much more).
'direction' becomes 'e' (E is an example).
'pep' going around 'e' is 'PEEP'.
Can you help me to learn more?
(Other definitions for peep that I've seen before include "Spy" , "Look furtively or voyeuristically" , "Large underground chamber" , "Peek" , "Look secretly" .)