John (originally Reg) altered to Len (5)
I believe the answer is:
elton
'john' is the definition.
(musician Elton John)
'altered to len' is the wordplay.
'altered' indicates anagramming the letters.
'to'+'len'='tolen'
'tolen' is an anagram of 'ELTON'.
'originally reg' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for elton that I've seen before include "Sir - - John, singer songwriter" , "Let on he's John, the singer" , "Ben --; -- John" , "..... Jonn (musician)" , "One John" .)