Against reactionary work by academic from down under (10)
I believe the answer is:
antipodean
'from down under' is the definition.
(I've seen this before)
'against reactionary work by academic' is the wordplay.
'against' becomes 'anti' (I've seen this before).
'reactionary' says the letters should be written in reverse (reactions happen in the opposite direction).
'work by' becomes 'op' (common abbreviation for 'opus'. I am not sure about the 'by' bit.).
'academic' becomes 'dean ' (I've seen this before).
'op' reversed gives 'po'.
'anti'+'po'+'dean'='ANTIPODEAN'
(Other definitions for antipodean that I've seen before include "Brit's description of Aussie" , "extreme point" , "from NZ?" , "as far away as possible" , "Exactly opposite" .)