Doctor tries second test (5)
I believe the answer is:
resit
'second test' is the definition.
(sitting an exam for a second time)
'doctor tries' is the wordplay.
'doctor' indicates anagramming the letters.
'tries' is an anagram of 'RESIT'.
(Other definitions for resit that I've seen before include "Take examination again" , "second exam" , "take another exam" , "Repeat (an exam)" , "Student's test" .)