The penniless possess nothing (3)
I believe the answer is:
nil
'nothing' is the definition.
('nil' can be a synonym of 'nothing')
'the penniless possess' is the wordplay.
'possess' indicates a hidden word.
'NIL' is hidden in the letters of 'penniless'.
(Other definitions for nil that I've seen before include "Lacking value" , "0" , "none" , "Nothing, no score" , "Nothing, zero" .)