Hard to believe suspect is loaded for the most part (8)
I believe the answer is:
doubtful
'hard to believe' is the definition.
(thesaurus)
'suspect is loaded for the most part' is the wordplay.
'suspect' becomes 'doubt' (doubting is a kind of suspecting).
'is loaded' becomes 'full' ('full' can be similar in meaning to 'loaded').
'for the most part' means to remove the last letter.
'full' with its final letter taken off is 'ful'.
'doubt'+'ful'='DOUBTFUL'
(Other definitions for doubtful that I've seen before include "Dubious" , "Vacillating" , "Uncertain, unlikely" , "suspect" , "Flout Dub, being undecided" .)