Unhappiness in party over any number wanting shelter (10)
I believe the answer is:
discontent
'unhappiness' is the definition.
(discontent is a kind of unhappiness)
'party over any number wanting shelter' is the wordplay.
'party' becomes 'disco' (dance party).
'over' means one lot of letters go next to another (in a down clue, some letters go over others).
'any number' becomes 'n' (mathematical symbol for an unspecified number).
'wanting' means one lot of letters go next to another.
'shelter' becomes 'tent' (a tent is a kind of shelter).
'disco'+'n'+'tent'='DISCONTENT'
'in' is the link.
(Other definitions for discontent that I've seen before include "Bad feelings" , "blighted Gloucester's winter" , "Now is the winter of our - (Rich.III)" , "Now is the winter of our - (Richard III)" , "Unhappiness" .)