A little short in cunning (8)
I believe the answer is:
slightly
'a little' is the definition.
(I've seen this before)
'short in cunning' is the wordplay.
'short' becomes 'light' (similar in meaning).
'in' indicates putting letters inside.
'cunning' becomes 'sly' (synonyms).
'light' inserted into 'sly' is 'SLIGHTLY'.
(Other definitions for slightly that I've seen before include "Quite!" , "Mildly" , "a tiny bit" , "To a small degree" , "To a small extent" .)