Register one with lease? Not initially (5)
I believe the answer is:
enter
'register' is the definition.
(I know that register is a kind of enter)
'one with lease? not initially' is the wordplay.
'one' becomes 'er' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'with' is a charade indicator (letters next to each other).
'lease?' becomes 'rent' ('rent' can be a synonym of 'lease').
'not initially' means to remove the first letter.
'rent' with its initial letter taken away is 'ent'.
'er' after 'ent' is 'ENTER'.
Can you help me to learn more?
(Other definitions for enter that I've seen before include "Come in - penetrate - record" , "Register formally as a participant" , "Come or go in" , "Gain access to" , "put in for a race" .)