Curious figures — and one unknown (4)
I believe the answer is:
nosy
'curious' is the definition.
(similar in meaning)
'figures and one unknown' is the wordplay.
'figures' becomes 'nos' (no is short for number ie a figure).
'and' means one lot of letters go next to another.
'one unknown' becomes 'y' (algebra).
'nos'+'y'='NOSY'
(Other definitions for nosy that I've seen before include "Inclined to poke into other people's affairs" , "Over-inquisitive" , "Intrusive in nasal way" , "Showing too much curiosity about people's affairs" , "Inquisitive in nasal manner" .)