Junkie using very large cup, one breaking law (6)
I believe the answer is:
addict
'junkie' is the definition.
(I've seen this before)
'very large cup one breaking law' is the wordplay.
'very large cup' becomes 'DD' (DD is a bra cup size).
'one' becomes 'i' (Roman numeral).
'breaking' indicates putting letters inside.
'law' becomes 'act' (I've seen this before).
'dd'+'i'='ddi'
'ddi' inserted within 'act' is 'ADDICT'.
'using' acts as a link.
(Other definitions for addict that I've seen before include "Habitual drug user" , "Victim of habit" , "Slave to a chemical substance" , "Person compulsively given to taking drugs" , "Fanatical devotee" .)