One-sided police lie naturally, but not extremely libellously (10)
I believe the answer is:
unilateral
'one-sided' is the definition.
('unilateral' can be a synonym of 'one-sided')
'police lie naturally but not extremely libellously' is the wordplay.
I cannot really see how this works, but
'not' could be 'un' (I've seen this in other clues) and 'un' is found within the answer.
This explanation may well be incorrect...
(Other definitions for unilateral that I've seen before include "affecting only one person out of several" , "Done only by one side" , "One-sided" .)