Sieve - tax (6)
I believe the answer is:
strain
I believe this is a double definition.
'sieve' is the first definition.
(I know that sieve can be written as strain)
'tax' is the second definition.
(both can mean to demand a lot from)
(Other definitions for strain that I've seen before include "Family - tension" , "Pressure - filter" , "Twist" , "Exert oneself to the utmost" , "Sieve; breed" .)