Tries unsuccessfully to take test again (5)
I believe the answer is:
resit
'test again' is the definition.
'resit' can be an answer for 'test' (I have seen 'Second test' mean 'resit' so perhaps 'test' could also mean 'resit'). I'm not certain of the 'again' bit.
'tries unsuccessfully' is the wordplay.
'tries' anagrams to 'RESIT'.
But, I'm not clear how the anagram is indicated.
'to take' is the link.
Can you help me to learn more?
(Other definitions for resit that I've seen before include "later exam" , "Repeat (an exam)" , "Retake exam" , "take another exam" , "Exam taken again" .)