Perhaps 3 in 1,000? (3)
I believe the answer is:
ink
'perhaps 3' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'in 1000?' is the wordplay.
'1000?' becomes 'k' ('K' can be a synonym of '1000').
'in'+'k'='INK'
(Other definitions for ink that I've seen before include "Printer's medium" , "Writing liquid" , "Squid's means of concealment" , "Writing medium" , "Cuttlefish defence" .)