Remorseless boxer, Ali, dancing, keeping mostly fresh (10)
I believe the answer is:
inexorable
'remorseless' is the definition.
(thesaurus)
'boxer ali dancing keeping mostly fresh' is the wordplay.
'dancing' indicates an anagram.
'keeping' is an insertion indicator (some letters kept inside).
'mostly' means to remove the last letter (most but not all of the word).
'fresh' becomes 'new' (synonyms).
'boxer'+'ali'='boxerali'
'boxerali' with letters rearranged gives 'ixorable'.
'new' with its last letter taken away is 'ne'.
'ixorable' going around 'ne' is 'INEXORABLE'.
(Other definitions for inexorable that I've seen before include "Relentless" , "Impossible to stop" , "Unstoppable" , "Inescapable" , "Unrelenting" .)