Appear again united — on King Charles carries! (5)
I believe the answer is:
recur
'appear again' is the definition.
(I've seen this in another clue)
'united on king charles carries' is the wordplay.
'united' becomes 'U' (abbreviation).
'on' becomes 're' ('on' can be mean 'with reference to').
'king charles' becomes 'CR' (Carolus Rex - King Charles in Latin).
'carries' indicates putting letters inside.
're'+'cr'='recr'
'u' going into 'recr' is 'RECUR'.
(Other definitions for recur that I've seen before include "Persist" , "expected to return" , "come back to one's mind" , "Return to mind" , "Be repeated" .)