Review of Alien not up for discussion (6)
I believe the answer is:
debate
'discussion' is the definition.
(debate is a kind of discussion)
'review of alien not up' is the wordplay.
'review of' says the letters should be written backwards.
'alien' becomes 'et' (extra terrestrial).
'not up' becomes 'abed' (I've seen this before).
'et'+'abed'='etabed'
'etabed' reversed gives 'DEBATE'.
'for' acts as a link.
(Other definitions for debate that I've seen before include "Formal discussion through argument" , "Contend" , "Public discussion, perhaps a contest" , "Hold public discussion about" , "Discuss (in parliament)" .)