Want number associated with topless act (4)
I believe the answer is:
need
'want' is the definition.
('need' can be a synonym of 'want')
'number associated with topless act' is the wordplay.
'number' becomes 'n' (**).
'associated with' says to put letters next to each other.
'topless' means to remove the first letter.
'act' becomes 'deed' (deed is a kind of act).
'deed' with its first letter taken off is 'eed'.
'n'+'eed'='NEED'
(Other definitions for need that I've seen before include "Lack - demand" , "What's lacking" , "privation" , "Necessity" , "Call" .)