The overwhelming emotion produced by politician welcoming one gangster on white horse (5,4)
I believe the answer is:
tidal wave
'the overwhelming emotion' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'politician welcoming one gangster on white horse' is the wordplay.
'politician welcoming' becomes 'tid' (I can't explain this - if you can you should give a lot more credence to this answer).
'one gangster' becomes 'al' (Al Capone).
'on' says to put letters next to each other.
'white horse' becomes 'wave' (I've seen this before).
'tid'+'al'+'wave'='TIDAL WAVE'
'produced by' is the link.
Can you help me to learn more?
(Other definitions for tidal wave that I've seen before include "Huge movement of public opinion" , "natural disaster?" , "potential disaster, naturally" , "Huge sea surge" , "Large movement (of water or public opinion)" .)