Tries unsuccessfully to take exam again (5)
I believe the answer is:
resit
'take exam again' is the definition.
(sit an exam again)
'tries unsuccessfully' is the wordplay.
'tries' anagrams to 'RESIT'.
Though, I'm not sure how this is indicated.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for resit that I've seen before include "Repeat (an exam)" , "second try at test" , "Enquiring intrusively (into person's affairs)" , "Take examination again" , "tested again" .)