Held back by extremely fit one-armed brief (6)
I believe the answer is:
notify
'brief' is the definition.
(thesaurus)
'held back by extremely fit one-armed' is the wordplay.
'back by' says the letters should be written in reverse.
'held' indicates a hidden word.
'yfiton' is hidden in the letters of 'extremely fit one armed'.
'yfiton' written backwards gives 'NOTIFY'.
(Other definitions for notify that I've seen before include "Officially inform" , "Make known" , "Advise, apprise" , "'Advise, tell about (6)'" , "No one" .)