Essential gold medallist drops head (5)
I believe the answer is:
inner
'essential' is the definition.
(thesaurus)
'gold medallist drops head' is the wordplay.
'gold medallist' becomes 'winner'.
'drops head' means to remove the first letter (I've seen 'dropping head' mean this).
'winner' with its initial letter taken off is 'INNER'.
(Other definitions for inner that I've seen before include "Opposite of outer" , "- - sanctum" , "This part of city is at the old centre" , "Annulus nearest bull" , "personal" .)