Below part of certain fragment (5)
I believe the answer is:
infra
'below' is the definition.
(synonyms)
'part of certain fragment' is the wordplay.
'part of' says the answer is hidden in the clue.
'INFRA' is hidden within 'certain fragment'.
(Other definitions for infra that I've seen before include "Latin adverb meaning below or further on in book" , "Dig is demeaning" , "Below (as prefix)" , "appearing later in book?" , "lower down" .)