For each upset select man changing substitutes (12)

'substitutes' is the definition.
(replacement is a kind of substitute)

'for each upset select man changing' is the wordplay.
'for each' becomes 'per' (I've seen this before).
'upset' says the letters should be written in reverse.
'changing' indicates anagramming the letters.
'per' written backwards gives 'rep'.
'select'+'man'='selectman'
'selectman' is an anagram of 'lacements'.
'rep'+'lacements'='REPLACEMENTS'