Vigilant owner ultimately enters a rental property (5)
I believe the answer is:
alert
'vigilant' is the definition.
(similar in meaning)
'owner ultimately enters a rental property' is the wordplay.
'ultimately' says to take the final letters.
'enters' is an insertion indicator.
'rental property' becomes 'let' (I've seen this in other clues).
The final letter of 'owner' is 'r'.
'a'+'let'='alet'
'r' inserted inside 'alet' is 'ALERT'.
(Other definitions for alert that I've seen before include "Vigilant, watchful" , "Watchful, sharp" , "Warning signal" , "sharp-witted" , "Watchful and bright" .)