Numbers unknown to the overly inquisitive (4)
I believe the answer is:
nosy
'the overly inquisitive' is the definition.
(I've seen this in another clue)
'numbers unknown' is the wordplay.
'numbers' becomes 'nos' ('no' is an abbreviation for number).
'unknown' becomes 'y' (algebra).
'nos'+'y'='NOSY'
'to' acts as a link.
(Other definitions for nosy that I've seen before include "Over-inquisitive" , "Intrusive in nasal way" , "Inclined to poke into other people's affairs" , "Showing too much curiosity about people's affairs" , "Offensively curious or inquisitive" .)