Clue: A little drunk now, number 3 in maths? (7)
Answer: UNKNOWN
Explanation: Definition: 3 in maths?
Hidden in [*a little*] "dr**UNK NOW N**umber" 
 
 
*The 3 in the clue refers to 3d, [the fourth of] (Jul)Y, and the letter y is often used as an unknown in algebra, geometry or trigonometry.