It's testament to the extremes of Winston Spencer-Churchill (4)
I believe the answer is:
will
'it's testament' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for will that I've seen before include "Resolve - testament" , "Written directions for disposal of estate after death" , "Final disposition" , "A traditional question from the floor?" , "Intention, desire" .)