Key in beginning withheld from tenant (5)
I believe the answer is:
enter
'key in' is the definition.
(both can mean to type in with a keyboard)
'beginning withheld from tenant' is the wordplay.
'beginning withheld from' means to remove the first letter (the letter which begins the word is removed).
'tenant' becomes 'renter' (synonyms).
'renter' with its first letter taken off is 'ENTER'.
(Other definitions for enter that I've seen before include "Go in - set down" , "Record - stage direction - come in" , "Fill in" , "Move into" , "Infiltrate" .)