On final letter coming in, fiddling emperor takes digits one to nine (3-4)
I believe the answer is:
non-zero
'takes digits one to nine' is the definition.
(not using the digit zero)
'on final letter coming in fiddling emperor' is the wordplay.
'final letter' becomes 'Z'.
'coming in' is an insertion indicator.
'fiddling emperor' becomes 'nero' (emperor who supposedly fiddled as Rome burned).
'on'+'z'='onz'
'onz' going within 'nero' is 'NON-ZERO'.
(Other definitions for non-zero that I've seen before include "Either positive or negative" , "Having a positive or negative value, ie anything not equating to nought (mathematics)" .)