Sounds like there's lots in this treaty (4)
I believe the answer is:
pact
'this treaty' is the definition.
(synonyms)
'sounds like there's lots in' is the wordplay.
'sounds like there' becomes 'pat' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'lots' becomes 'c'.
'in' means one lot of letters goes inside another.
'pat' enclosing 'c' is 'PACT'.
Can you help me to learn more?
(Other definitions for pact that I've seen before include "'Accord, treaty (4)'" , "Agreement, bargain" , "Agreement, covenant" , "deal" , "Alliance" .)