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' is an anagram indicator.
'keeping' means one lot of letters goes inside another (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 off is 'ne'.
'ixorable' enclosing 'ne' is 'INEXORABLE'.
(Other definitions for inexorable that I've seen before include "Relentless" , "Impossible to stop" , "Unstoppable" , "Inescapable" , "Unrelenting" .)