Said to swap characters at either end of this stage (4)
I believe the answer is:
dais
'characters at either end of this stage' is the definition.
'dais' can be an answer for 'stage' (type of platform or stage). I'm not certain of the remainder of the definition.
'said to swap' is the wordplay.
'to swap' indicates an anagram (I've seen 'swaps' mean this).
'said' anagrammed gives 'DAIS'.
Can you help me to learn more?
(Other definitions for dais that I've seen before include "Throne platform" , "stage" , "Low platform at end of hall" , "stand" , "Welshman's" .)