In agreement negotiating is done (6)
I believe the answer is:
onside
'in agreement' is the definition.
(I've seen this before)
'negotiating is done' is the wordplay.
'negotiating' indicates anagramming the letters (I've seen 'negotiated' mean this).
'is'+'done'='isdone'
'isdone' is an anagram of 'ONSIDE'.
(Other definitions for onside that I've seen before include "(In football) not in forbidden zone" , "in a good position" , "in a position to score?" , "Legally positioned at football etc" , "in a permitted position in sport" .)