Isn't regularly on guard outside, one's deduced (9)
I believe the answer is:
inference
'one's deduced' is the definition.
(something deduced or inferred)
'isn't regularly on guard outside' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'on' becomes 're' ('on' can be mean 'with reference to').
'guard' becomes 'fence' (I've seen this before).
'outside' means one lot of letters goes inside another.
The alternate letters of 'isnt' are 'in'.
're' put within 'fence' is 'ference'.
'in'+'ference'='INFERENCE'
(Other definitions for inference that I've seen before include "Conclusion reached on the basis of evidence and reasoning" , "Deduction from evidence" , "Deduction, conclusion" , "Niece fern (anag.)" .)