One who won't drink and drive, in practice (3,3)
I believe the answer is:
dry run
'practice' is the definition.
(I know that dry run is a type of practice)
'one who won't drink and drive' is the wordplay.
'one who won't drink' becomes 'dry' (dry can mean someone who doesn't drink alcohol).
'and' means one lot of letters go next to another.
'drive' becomes 'run' (synonyms).
'dry'+'run'='DRY RUN'
'in' acts as a link.
(Other definitions for dry run that I've seen before include "Kind of rehearsal like thirsty trot" , "event staged by AA?" , "Thirsty sort of rehearsal?" , "Rehearsal, test" , "Dummy trial on thirsty trot?" .)