In a clumsy way, one gets plenty wrong (7)
I believe the answer is:
ineptly
'in a clumsy way' is the definition.
(inept can mean clumsy or awkward)
'one gets plenty wrong' is the wordplay.
'one' becomes 'I' (Roman numeral).
'gets' is a charade indicator (letters next to each other).
'wrong' is an anagram indicator.
'plenty' is an anagram of 'neptly'.
'i'+'neptly'='INEPTLY'
(Other definitions for ineptly that I've seen before include "without finesse" , "In a maladroit manner" , "Awkwardly, clumsily" , "In an incompetent manner" .)