Order remark about fellow dunderhead (11)
I believe the answer is:
commandment
'order' is the definition.
(thesaurus)
'remark about fellow dunderhead' is the wordplay.
'remark' becomes 'comment' (synonyms).
'about' is an insertion indicator.
'fellow dunderhead' becomes 'mand' (I can't explain this - if you can you should give a lot more credence to this answer).
'comment' going around 'mand' is 'COMMANDMENT'.
Can you help me to learn more?
(Other definitions for commandment that I've seen before include "Order (one of ten)" , "Divine rule" .)