Come up against an objection (4)
I believe the answer is:
abut
'come 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 "Touch (side of next building)" , "Adjoin, press up against" , "Tuba (anag)" , "Cease (from activity)" , "Touch upon" .)