Physicist I introduced to rising wit (5)
I believe the answer is:
dirac
'physicist' is the definition.
(I've seen this before)
'i introduced to rising wit' is the wordplay.
'introduced to' is an insertion indicator.
'rising' shows that the letters should be reversed in order (going up in a down clue).
'wit' becomes 'card' (synonyms).
'card' reversed gives 'drac'.
'i' inserted within 'drac' is 'DIRAC'.
(Other definitions for dirac that I've seen before include "English physicist" , "constant originator" , "Nobel Prize winner" , "Paul -, physicist; I'd arc (anag.)" , "Scientist" .)