Finds fault with blockhead weak at school (6)
I believe the answer is:
blames
'finds fault with' is the definition.
('blame' can be a synonym of 'find fault with')
'blockhead weak at school' is the wordplay.
'blockhead' becomes 'b' (head letter of 'block').
'weak' becomes 'lame' ('lame' can be similar in meaning to 'weak'**).
'at' means one lot of letters go next to another.
'school' becomes 's'.
'b'+'lame'+'s'='BLAMES'
(Other definitions for blames that I've seen before include "Deems to be responsible for a misdeed" , "Considers someone responsible for" , "Holds responsible for fault or misdeed" .)