Hard-working and conscientious (8)
I believe the answer is:
'hard-working and conscientious' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for diligent that I've seen before include "Painstaking - I tingled (anag)" , "I tingled, being hard-working and conscientious" , "Putting care and effort into work" , "Attentive and busy" , "Taking trouble" .)