To give a fathead credit is stupid (5)
I believe the answer is:
'is stupid' is the definition.
(I've seen this before)
'to give a fathead credit' is the wordplay.
I cannot really see how this works, but
'a' is present in the answer.
'credit' could be 'cr' (abbreviation) and 'cr' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for crass that I've seen before include "Coarse" , "Showing gross lack of sensitivity" , "Indelicate" , "Asinine" , "Grossly insensitive" .)