Frost on the third of September? That's first-rate! (5)
I believe the answer is:
prime
'first-rate' is the definition.
(I've seen this in another clue)
'frost on the third of september?' is the wordplay.
'frost' becomes 'rime' (rime is a type of frost).
'on' is a charade indicator (letters next to each other).
'the third of september?' becomes 'p' (3rd letter of 'september').
'rime' after 'p' is 'PRIME'.
'that's' is the link.
(Other definitions for prime that I've seen before include "(Of number) divisible only by itself and one" , "Main; make ready" , "Original" , "Prepare to fire - first" , "Chief or most important, like a minister" .)