One prompts backward solicitor: speak! (4,2,3)
I believe the answer is:
spit it out
'speak' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'one prompts backward solicitor' is the wordplay.
'one' becomes 'I' (Roman numeral).
'prompts' becomes 'tips' (I am not sure about this - if you are sure you should believe this answer much more).
'backward' shows that the letters should be reversed in order.
'solicitor' becomes 'tout' (someone who solicits business).
'i'+'tips'='itips'
'itips' backwards is 'spiti'.
'spiti'+'tout'='SPIT IT OUT'
Can you help me to learn more?
(Other definitions for spit it out that I've seen before include "Come on, say what you think - expectorate" , "Say what one has to say" , "Say it quickly - even expectorate" , "Come on, say what's on your mind - expectorate if you wish" , "Go on!" .)