Disincentive of French shelter - stray inside (9)
I believe the answer is:
deterrent
'disincentive' is the definition.
(I've seen this before)
'of french shelter stray inside' is the wordplay.
'of french' becomes 'de' ('of' in French).
'shelter' becomes 'tent' (canvas shelter).
'stray' becomes 'err' (to err is to stray or go wrong).
'inside' is an insertion indicator.
'tent' enclosing 'err' is 'terrent'.
'de'+'terrent'='DETERRENT'
(Other definitions for deterrent that I've seen before include "It restrains" , "Something designed to be off-putting" , "Discouragement, disincentive" , "It acts to discourage" , "Nuclear weapons allegedly" .)