To make sense of Sun puzzle, many go on and on (10)
I believe the answer is:
unscramble
'to make sense of' is the definition.
'sun puzzle many go on and on' is the subsidiary indication.
'puzzle' indicates an anagram.
'many' becomes 'c' (C is the Roman numeral for 100).
'go on and on' becomes 'ramble'.
'sun' with letters rearranged gives 'uns'.
'uns'+'c'+'ramble'='unscramble'
(Other definitions for unscramble that I've seen before include "Restore to a readable state" , "Restore to readability" .)