Chop up, making many a nice mess (5)
I believe the answer is:
mince
'chop up' is the definition.
(mincing is a kind of chopping up)
'many a nice mess' is the wordplay.
'many' becomes 'm' (Roman numeral for a thousand).
'mess' indicates an anagram.
'nice' anagrammed gives 'ince'.
'm'+'ince'='MINCE'
'making' acts as a link.
(Other definitions for mince that I've seen before include "Shredded beef" , "meat in small portions" , "Grind up (meat)" , "Simper" , "Grind (food)" .)