Practice that's bad for a doctor (5)
I believe the answer is:
drill
'practice' is the definition.
(synonyms)
'bad for a doctor' is the wordplay.
'bad' becomes 'ill' (associated in meaning).
'for' says to put letters next to each other (I've seen this in other clues).
'a doctor' becomes 'dr' ('Dr' can be a synonym of 'doctor').
'ill' put after 'dr' is 'DRILL'.
'that's' is the link.
(Other definitions for drill that I've seen before include "Twill-weave cotton cloth" , "Exercise - bore a hole" , "Routine" , "Dental assistant" , "Stout cotton cloth" .)