About to enter a shoe (4)
I believe the answer is:
clog
'a shoe' is the definition.
(I know that clog is a type of shoe)
'about to enter' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'to enter' becomes 'log' (log can mean to record or enter).
'c'+'log'='CLOG'
(Other definitions for clog that I've seen before include "Obstruct the wooden shoe" , "Wooden shoe; choke up" , "Block up - type of shoe" , "Block with wooden-soled shoe?" , "Shoe with a thick wooden sole" .)