Was wrong to make some further reduction (5)
I believe the answer is:
erred
'was wrong' is the definition.
(I've seen this in another clue)
'some further reduction' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'ERRED' is hidden in the letters of 'further reduction'.
'to make' is the link.
(Other definitions for erred that I've seen before include "Made a mistake, was wrong" , "Gone astray!" , "Sinned" , "slipped up" , "Made mistake, was incorrect" .)