Some digit I nudged up, whole number (4)
I believe the answer is:
unit
'whole number' is the definition.
(unit is a kind of whole number)
'some digit i nudged up' is the wordplay.
'some' indicates a hidden word (some parts of the word are used).
'up' is a reversal indicator (in down clue: letters go upwards).
'tinu' is hidden in the letters of 'digit i nudged'.
'tinu' in reverse letter order is 'UNIT'.
(Other definitions for unit that I've seen before include "Single item, maybe part of a set" , "Constituent" , "A single self-contained item" , "Part of a kit" , "Single undivided whole" .)