Poet in argument over merit (10)
I believe the answer is:
wordsworth
'poet' is the definition.
(William Wordsworth, 1770-1850)
'argument over merit' is the wordplay.
'argument' becomes 'words' (to have words with someone is to argue with them).
'over' means one lot of letters go next to another.
'merit' becomes 'worth' (merit can mean worth or value).
'words'+'worth'='WORDSWORTH'
'in' acts as a link.
(Other definitions for wordsworth that I've seen before include "writer" , "Poet (of the 20s)" , "Romantic poet" .)