Had been the cutter-up (3)
I believe the answer is:
was
'had been the' is the definition.
I can't tell whether this defines the answer.
'cutter-up' is the wordplay.
'cutter' becomes 'saw' (I've seen this in other clues).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'saw' reversed gives 'WAS'.
Can you help me to learn more?
(Other definitions for was that I've seen before include "Is no longer" , "Existed in the past" , "(I/he/she/it) existed" , "'Is, in the past (3)'" , "(He/she) existed" .)