Plan Ian describes entering apartment initially (4)
I believe the answer is:
idea
'plan' is the definition.
(both can mean an intention)
'ian describes entering apartment initially' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'ian describes entering apartment' is 'IDEA'.
(Other definitions for idea that I've seen before include "Fancy" , "Suggestion" , "Purpose" , "Flash of inspiration?" , "A suspicion of" .)