Law lord who ran to catch duke (4)
I believe the answer is:
code
'law' is the definition.
(thesaurus)
'lord who ran to catch duke' is the wordplay.
I cannot quite understand how this works, but
'catch' could be 'cod' (type of fish) and 'cod' is found in the answer.
A single letter 'e' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for code that I've seen before include "Secret language" , "Collection of laws" , "Playfair is one" , "System of ethics - cipher" , "Cipher - set of principles" .)