Deal using diplomacy to secure managed end of hostilities (8)
I believe the answer is:
transact
'deal' is the definition.
(I know that deal is a kind of the transact)
'diplomacy to secure managed end of hostilities' is the wordplay.
'diplomacy' becomes 'tact' (tact can mean diplomacy or sensitivity).
'to secure' is an insertion indicator (inserted letters are secured inside).
'managed' becomes 'ran' (both can mean organised or was in charge of).
'end of' indicates one should take the final letters.
The last letter of 'hostilities' is 's'.
'ran'+'s'='rans'
'tact' placed around 'rans' is 'TRANSACT'.
'using' is the link.
(Other definitions for transact that I've seen before include "Conduct trade" , "Manage" , "handle" , "Buy or sell" , "Conduct business, negotiate" .)