City replaces masthead in general correspondence (10)
I believe the answer is:
comparison
'correspondence' is the definition.
(thesaurus)
'city replaces masthead in general' is the wordplay.
I cannot quite understand how this works, but
'city' could be 'paris' (Paris is an example) and 'paris' is found within the answer.
'masthead' could be 'm' (head letter of 'mast') and 'm' is found in the answer.
The remaining letters 'coon' is a valid word which might be clued in a way I don't see.
This may be the basis of clue (or it may be nonsense).
(Other definitions for comparison that I've seen before include "The drawing of parallels" , "Analogy; likeness" , "Process of identifying differences" , "Similarity" , "In contrast" .)