At one point, you start to panic for sure (3)
I believe the answer is:
yep
'sure' is the definition.
(I've seen this in another clue)
'at one point you' is the wordplay.
'at' means one lot of letters go next to another.
'one point' becomes 'p' (abbreviation).
'you' becomes 'ye' (historical version of 'you').
'p' put after 'ye' is 'YEP'.
'start to panic for' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for yep that I've seen before include "informal approval" , "Affirmative (US colloq.)" , "Positive reaction" , "Correct" .)