Below part of Thunderball (5)
I believe the answer is:
under
'below' is the definition.
(synonyms)
'part of thunderball' is the wordplay.
'part of' indicates a hidden word.
'UNDER' is hidden in the letters of 'thunderball'.
(Other definitions for under that I've seen before include "See 7" , "One prepared to take over from leading actor" , "Inferior" , "Dylan Thomas wrote '. . . . . Milk Wood'" , "Being short" .)