Used to be nearly all sawn up (3)
I believe the answer is:
was
'used to be' is the definition.
(eg 'he was...' means 'he used to be...')
'nearly all sawn up' is the wordplay.
'nearly all' means to remove the last letter.
'up' is a reversal indicator (in down clue: letters go upwards).
'sawn' with its final letter taken away is 'saw'.
'saw' reversed gives 'WAS'.
(Other definitions for was that I've seen before include "Existed in the past" , "(He/she/it) existed" , "Is no longer" , "(I/he/she/it) existed" , "'Is, in the past (3)'" .)