Treaty in the bag, one might say (4)
I believe the answer is:
pact
'treaty' is the definition.
(synonyms)
'bag one might say' is the wordplay.
I cannot quite see how this works, but
'one' could be 'a' (a thing is one thing) and 'a' is found in the answer.
The remaining letters 'pct' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'in the' is the link.
Can you help me to learn more?
(Other definitions for pact that I've seen before include "Alliance" , "International treaty" , "Treaty, formal agreement" , "deal" , "Agreement, covenant" .)