Stop a number going in as usual (9)
I believe the answer is:
prevalent
'usual' is the definition.
(thesaurus)
'stop a number going in' is the wordplay.
'stop' becomes 'prevent' (I've seen this in other clues).
'a number going' becomes 'al' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'prevent' enclosing 'al' is 'PREVALENT'.
'as' acts as a link.
Can you help me to learn more?
(Other definitions for prevalent that I've seen before include "Common" , "Commonplace" , "Widespread - current" , "powerful" , "Ruling" .)