A partner placed inside is cut off (8)
I believe the answer is:
amputate
'is cut off' is the definition.
(I've seen this in another clue)
'a partner placed inside' is the wordplay.
'a partner' becomes 'amate' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'placed' becomes 'put' (I've seen this in other clues).
'inside' indicates putting letters inside.
'amate' placed around 'put' is 'AMPUTATE'.
Can you help me to learn more?
(Other definitions for amputate that I've seen before include "Cut off (a limb)" , "In theatre, remove" , "Maybe disarm" , "Surgically remove all or part of limb" , "Cut off an arm or a leg" .)