Put down regular contribution required (6)
I believe the answer is:
subdue
'put down' is the definition.
(I've seen this before)
'regular contribution required' is the wordplay.
'regular contribution' becomes 'sub' (I can't justify this - if you can you should give a lot more credence to this answer).
'required' becomes 'due' (due can mean required or expected).
'sub'+'due'='SUBDUE'
Can you help me to learn more?
(Other definitions for subdue that I've seen before include "Overcome, bring under control" , "Conquer" , "Moderate" , "Overcome, quieten" , "Force to yield, tame" .)