In rehab I treated addiction (5)
I believe the answer is:
habit
'addiction' is the definition.
(I've seen this before)
'in rehab i treated' is the wordplay.
'in' indicates a hidden word.
'HABIT' is hidden in the letters of 'rehab i treated'.
(Other definitions for habit that I've seen before include "Usual practice; garment" , "addictive practice" , "nun gets into" , "It's second nature" , "It's worn in order" .)