Go without lunch, say, in agreement (6)
I believe the answer is:
treaty
'agreement' is the definition.
(I know that agreement can be written as treaty)
'go without lunch say' is the wordplay.
'go' becomes 'try' (both can mean an attempt).
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'lunch say' becomes 'eat' (lunching is a kind of eating).
'try' enclosing 'eat' is 'TREATY'.
'in' is the link.
(Other definitions for treaty that I've seen before include "Negotiation" , "Contract between states" , "something of Rome or Versailles?" , "Compact; concordat" , "Contract, pact" .)