He's posh, and certainly no blockhead! (3)
I believe the answer is:
nob
'he's posh and certainly' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'no blockhead' is the wordplay.
'blockhead' becomes 'b' (head letter of 'block').
'no'+'b'='NOB'
Can you help me to learn more?
(Other definitions for nob that I've seen before include "A slang toff - c'est bon" , "A slangy person of high rank - c'est bon" , "A posh gent - c'est bon" , "Jack" , "Aristocrat" .)