Guard ate up remainder (6)
I believe the answer is:
defend
'guard' is the definition.
(thesaurus)
'ate up remainder' is the wordplay.
'ate' becomes 'fed' ('feed' can be a synonym of 'eat').
'up' is a reversal indicator (in down clue: letters go upwards).
'remainder' becomes 'end' ('end' can be a synonym of 'remainder').
'fed' in reverse letter order is 'def'.
'def'+'end'='DEFEND'
(Other definitions for defend that I've seen before include "Block, as in cricket" , "Champion" , "Protect against challenge or attack" , "Protect, fight back" , "Safeguard" .)