Was wrong in making further reductions (5)
I believe the answer is:
erred
'was wrong' is the definition.
(I've seen this before)
'in making further reductions' is the wordplay.
'in' indicates the answer is hidden within the clue.
'ERRED' is hidden in the letters of 'making further reductions'.
(Other definitions for erred that I've seen before include "was inaccurate" , "Made mistake, was incorrect" , "Sinned" , "Strayed from the straight and narrow" , "Went wrong" .)