Little devil by old shelter, unable to act (8)
I believe the answer is:
impotent
'unable to act' is the definition.
'little devil by old shelter' is the wordplay.
'little devil' becomes 'imp' (I've seen this before).
'by' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'shelter' becomes 'tent' (tent is a kind of shelter).
'imp'+'o'+'tent'='IMPOTENT'
(Other definitions for impotent that I've seen before include "Mon petit may be powerless" , "Useless" , "Powerless like mon petit" , "Helpless, powerless" , "Ineffective" .)