One who works their way to the top? (6)
I believe the answer is:
either
'one' is the definition.
(thesaurus)
'works their way to the top?' is the wordplay.
'works' is an anagram indicator (work can mean to twist or convulse).
'way' becomes 'e' (Compass direction - East).
'to the' becomes 't' (the is pronounced as a 't' sound in some dialects).
'top?' means to remove the first letter (I've seen 'top' mean this).
'their'+'e'+'t'='theiret'
'theiret' with its first letter taken away is 'heiret'.
'heiret' anagrammed gives 'EITHER'.
'who' is the link.
(Other definitions for either that I've seen before include "No matter which (of two)" , "Each of two; moreover" , "Her tie (anag.)" , "Choice of two" , "offered a choice" .)