Disciple — this person's held to be one (5)
I believe the answer is:
simon
'be one' is the definition.
The definition suggests a singular noun which matches the answer.
'disciple this person's held' is the wordplay.
'disciple' becomes 'son' (I've seen this in other clues).
'this person's' becomes 'im' (this person means the crossword setter themselves).
'held' indicates putting letters inside.
'son' placed around 'im' is 'SIMON'.
'to' acts as a link.
(Other definitions for simon that I've seen before include "- and Garfunkel" , "One of Christ's apostles" , "he says it's a game" , "Bolivar, de Montfort, Callow or Cowell?" , "Fellow" .)