Cuts fish by loch, adopting duke’s crooked posture (12)
I believe the answer is:
lopsidedness
'crooked posture' is the definition.
'cuts fish by loch' is the wordplay.
'cuts fish by' becomes 'lopsided' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'loch' becomes 'ness' (I've seen this before).
'lopsided'+'ness'='LOPSIDEDNESS'
'adopting duke's' is the link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Another definition for lopsidedness that I've seen is " Lack of symmetry".)