Exam did not start before the heartless retort (7)

I believe the answer is:

riposte

I'm a little stuck... Click here to teach me more about this clue!

'the heartless retort' is the definition.
'riposte' can be an answer for 'retort' ('riposte' can be a synonym of 'retort'). I'm unsure of the 'the heartless' bit.

'exam did not start before' is the wordplay.
'exam' becomes 'tripos' (tripos is a kind of exam).
'did not start' means to remove the first letter.
'before' becomes 'te' (I can't justify this - if you can you should give a lot more credence to this answer).
'tripos' with its first letter taken away is 'ripos'.
'ripos'+'te'='RIPOSTE'

Can you help me to learn more?

(Other definitions for riposte that I've seen before include "Sally" , "Quick answer" , "Return thrust or repartee" , "Fencer's counterattack" , "Quick and witty reply" .)

I've seen this clue in The Telegraph.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.