He, lacking guile, is callous (9)
I believe the answer is:
heartless
'is callous' is the definition.
(I've seen this before)
'he lacking guile' is the wordplay.
'lacking guile' becomes 'artless' (I've seen this in another clue).
'he'+'artless'='HEARTLESS'
(Other definitions for heartless that I've seen before include "Without pity or feeling, being defective in cardiac way" , "Uncaring" , "Callous" , "with no concern for others?" , "insensitive" .)