Campbell for one writer briefly entertaining a mystic syllable (5)
I believe the answer is:
naomi
'campbell for one' is the definition.
(UK model)
'writer briefly entertaining a mystic syllable' is the wordplay.
'writer' becomes 'nib' (something which writes).
'briefly' means to remove the last letter.
'entertaining' is an insertion indicator.
'mystic syllable' becomes 'om' (sacred syllable in many Sanskrit texts).
'nib' with its final letter taken off is 'ni'.
'a'+'om'='aom'
'ni' going around 'aom' is 'NAOMI'.
(Other definitions for naomi that I've seen before include "Girl's name, meaning "pleasant"" , "Relative of Ruth" , "woman's name" , "Biblical woman" , "mother- in-law" .)