Number brunette’s holding up (3)
I believe the answer is:
ten
'number' is the definition.
(ten is a kind of whole number)
'brunette's holding up' is the wordplay.
'holding' indicates the answer is hidden within the clue.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'net' is hidden in the letters of 'brunettes'.
'net' reversed gives 'TEN'.
(Other definitions for ten that I've seen before include "Very young" , "Roman numeral X" , "time" , "No of sides of a decagon" , "Figure" .)