Wants to be seen holding a number up (5)
I believe the answer is:
needs
'wants' is the definition.
(needing is a kind of wanting)
'seen holding a number up' is the wordplay.
'holding' indicates putting letters inside.
'a number' becomes 'd' (D is an example**).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'seen' enclosing 'd' is 'sdeen'.
'sdeen' written backwards gives 'NEEDS'.
'to be' acts as a link.
(Other definitions for needs that I've seen before include "What we require" , "Misses" , "Requirements" , "necessity once" , "Requires or lacks" .)