Kind but uncouth fellows nourish mad dog on the loose (5,8)
I believe the answer is:
rough diamonds
'kind but uncouth fellows' is the definition.
'nourish mad dog on the loose' is the wordplay.
'on the loose' indicates anagramming the letters.
'nourish'+'mad'+'dog'='nourishmaddog'
'nourishmaddog' anagrammed gives 'ROUGH DIAMONDS'.
(Other definitions for rough diamonds that I've seen before include "simple, honest geezers" , "More than one good bloke lacking social graces" .)