Hair preparation used by Nigella (3)
I believe the answer is:
gel
'hair preparation' is the definition.
(I've seen this before)
'used by nigella' is the wordplay.
'used by' indicates a hidden word.
'GEL' can be found hidden inside 'nigella'.
(Other definitions for gel that I've seen before include "Posh female" , "posh lass" , "Mousse" , "Take form" , "Solidify" .)