Assume for argument one is needed in support (5)
I believe the answer is:
posit
'assume for argument' is the definition.
'posit' can be an answer for 'assume' (I've seen this before). I am unsure of the 'for argument' bit.
'one is needed in support' is the wordplay.
'one' becomes 'i' (Roman numeral).
'is needed in' means one lot of letters goes inside another.
'support' becomes 'post' (post is a kind of support).
'i' placed within 'post' is 'POSIT'.
Can you help me to learn more?
(Other definitions for posit that I've seen before include "Dispose" , "Lay down or assume (as the basis for an argument)" , "advance" , "Propose" , "Assume as fact for argument's sake" .)