Notice how speaking becomes a terrible habit? (9)
I believe the answer is:
addiction
'a terrible habit?' is the definition.
'addiction' can be an answer for 'habit?' (drug habit). I am not certain of the 'a terrible' bit.
'notice how speaking' is the wordplay.
'notice' becomes 'ad' (short for advert).
'how speaking' becomes 'diction' (I am not sure about this - if you are sure you should believe this answer much more).
'ad'+'diction'='ADDICTION'
'becomes' is the link.
Can you help me to learn more?
(Other definitions for addiction that I've seen before include "Bug one's caught" , "hard to stop" , "Compulsive habit" , "Drug-habit" , "Something to kick" .)