Apply pressure to employ warders (3,3,6,2)
I believe the answer is:
put the screws on
'pressure to employ warders' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'apply' is the wordplay.
I cannot really see how this works, but
'apply' could be 'put' (putting is a kind of applying) and 'put' is located in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for put the screws on that I've seen before include "Demand to speak to warders by phone" , "Use pressure" .)