One picks up booze and ales: try rehab when drunk (12)
I believe the answer is:
'one picks up booze' is the definition.
(device which detects or picks up alcohol)
'ales try rehab when drunk' is the wordplay.
'when drunk' indicates an anagram (a drunken or disorderly form of the letters).
'alestryrehab' is an anagram of 'BREATHALYSER'.
'and' acts as a link.
(Other definitions for breathalyser that I've seen before include "Blood alcohol tester" , "Device to identify drunk drivers" , "Device for testing drivers (S, not Z)" , "Device used on suspected drunk drivers" , "Device to test level of alcohol consumed" .)