Just one snitch spilled the beans (9)
I believe the answer is:
singleton
'just one' is the definition.
'singleton' can be an answer for 'one' (I've seen this before). I am not certain of the 'just' bit.
'snitch spilled the beans' is the wordplay.
'snitch' becomes 'sing' (I've seen this in another clue).
'spilled the beans' becomes 'leton' (I've seen this in another clue).
'sing'+'leton'='SINGLETON'
Can you help me to learn more?
(Other definitions for singleton that I've seen before include "lonely heart, perhaps" , "one diamond, perhaps" , "Eg lonely heart" , "In bridge, a holding of one card only in a suit" , "person not in a long-term relationship?" .)