A rough figure, this person's breaking into property (8)
I believe the answer is:
estimate
'a rough figure' is the definition.
(I've seen this in another clue)
'this person's breaking into property' is the wordplay.
'this person's' becomes 'I'm' (('I am' - 'this person' means the crossword setter).
'breaking into' indicates putting letters inside.
'property' becomes 'estate' (estate is a kind of property).
'im' going into 'estate' is 'ESTIMATE'.
(Other definitions for estimate that I've seen before include "Calculate in approximate way" , "Statement giving the likely cost of a job" , "Gauge - approximation" , "Evaluate - approximation" , "Calculate approximately" .)