Go up against an objection (4)
I believe the answer is:
abut
'go up against' is the definition.
(I've seen this in another clue)
'an objection' is the wordplay.
'an' becomes 'a'.
'objection' becomes 'but' (as in 'no ifs, no buts').
'a'+'but'='ABUT'
(Other definitions for abut that I've seen before include "Adjoin at one end" , "Tuba (anag)" , "Join" , "Touch (side of next building)" , "Touch upon" .)