Agreement to attempt to catch swallow (6)
I believe the answer is:
treaty
'agreement' is the definition.
(treaty is a kind of agreement)
'attempt to catch swallow' is the wordplay.
'attempt' becomes 'try' (I've seen this before).
'to catch' is an insertion indicator.
'swallow' becomes 'eat' (I've seen this before).
'try' going around 'eat' is 'TREATY'.
'to' acts as a link.
(Other definitions for treaty that I've seen before include "Contract between states" , "International agreement" , "Pact, accord between states" , "Yatter (anag.)" , "Formal agreement between nations" .)