E.g. an ‘A’ written in hexameters (4)
I believe the answer is:
exam
'e g an a' is the definition.
(as in an A-level exam)
'written in hexameters' is the wordplay.
'written in' indicates the answer is hidden within the clue.
'EXAM' is hidden in the letters of 'hexameters'.
(Other definitions for exam that I've seen before include "Formal knowledge test" , "(Academic) test" , "Oral maybe" , "paper" , "Test (abbrev.)" .)