Swap a letter A and N, incorrectly (9)
I believe the answer is:
alternate
'swap' is the definition.
(thesaurus)
'a letter a and n incorrectly' is the wordplay.
'and' is a charade indicator (letters next to each other).
'incorrectly' indicates an anagram (I've seen this in other clues).
'letter'+'a'='lettera'
'lettera' after 'n' is 'nlettera'.
'nlettera' anagrammed gives 'lternate'.
'a'+'lternate'='ALTERNATE'
(Other definitions for alternate that I've seen before include "Interchange" , "Flip" , "Change from one thing to another by turns" , "Every second one of a series" , "go back and forth" .)