Stay quiet about attack in vessel (4,2)
I believe the answer is:
clam up
'stay quiet' is the definition.
Although both the answer and definition are verbs in their base form, I don't understand how they can define each other.
'about attack in vessel' is the wordplay.
'about attack' becomes 'lam' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'vessel' becomes 'cup' (cup is a type of vessel).
'lam' put into 'cup' is 'CLAM UP'.
Can you help me to learn more?
(Other definitions for clam up that I've seen before include "Refuse to speak" , "'Fall silent and refuse to talk (4,2)'" , "Stop giving out" , "Refuse to talk like large shellfish?" , "Keep mum" .)