Agreement is to make an effort to dine inside (6)
I believe the answer is:
treaty
'agreement' is the definition.
(treaty is a kind of agreement)
'make an effort to dine inside' is the wordplay.
'make an effort' becomes 'try' (I've seen this before).
'to dine' becomes 'eat' (I've seen this before).
'inside' means one lot of letters goes inside another.
'try' placed around 'eat' is 'TREATY'.
'is to' is the link.
(Other definitions for treaty that I've seen before include "Formal agreement between nations" , "Covenant" , "Pact, accord between states" , "International agreement" , "settlement" .)