### Clue: Bring up drink, keeping unknown under the boot, it may be (4,4)
### Answer: REAR AXLE 
 ### Explanation: Definition: under the boot, it may be
 REAR(bring up, eg. pets) + ALE(a drink) ***containing***(keeping) X(in mathematics, a symbol for an unknown quantity).
Answer:  The "it" that is found under the boot (or trunk in N. America) of a car perhaps,may be as not all cars have their boot at the rear.