Item to be kept secret? Mostly dull fool slipping up (8)
I believe the answer is:
password
'item to be kept secret?' is the definition.
'mostly dull fool slipping up' is the wordplay.
'mostly' means to remove the last letter.
'dull' becomes 'drowsy' (I've seen this before).
'fool' becomes 'sap' (synonyms).
'slipping up' says the letters should be written in reverse (in a down clue, letters go up).
'drowsy' with its final letter removed is 'drows'.
'drows'+'sap'='drowssap'
'drowssap' reversed gives 'PASSWORD'.
(Other definitions for password that I've seen before include "It's something to admit" , "that'll secure one's access" , "key" , "Requirement for logging in" , "Secret requirement for access" .)