Bang on entrance to palace at once, if it's unstable (11)
I believe the answer is:
pontificate
'bang' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'entrance to palace at once if it's unstable' is the wordplay.
'entrance to' suggests taking the first letters.
'unstable' is an anagram indicator.
The initial letter of 'palace' is 'p'.
'at'+'once'+'if'+'it'='atonceifit'
'atonceifit' with letters rearranged gives 'ontificate'.
'p'+'ontificate'='PONTIFICATE'
'on' acts as a link.
Can you help me to learn more?
(Other definitions for pontificate that I've seen before include "Office or term of Pope" , "Talk in a pompous and dogmatic way" , "Pope's position" , "sound off" , "Express one's opinions pompously" .)