Idea put forward as concession upset relations (5)
I believe the answer is:
posit
'idea put forward' is the definition.
I can't judge whether this defines the answer.
'concession upset relations' is the wordplay.
'concession' becomes 'sop' (sop is a kind of concession).
'upset' shows that the letters should be reversed in order.
'relations' becomes 'it' (both can mean sex).
'sop' backwards is 'pos'.
'pos'+'it'='POSIT'
'as' acts as a link.
Can you help me to learn more?
(Other definitions for posit that I've seen before include "Suggest" , "Put forward (as fact)" , "Put forward as basis for discussion" , "Propose" , "Assume as a fact or principle" .)