Uncertainties in new studbook not OK (6)
I believe the answer is:
doubts
'uncertainties' is the definition.
('doubt' can be a synonym of 'uncertainty')
'new studbook not ok' is the wordplay.
'new' indicates an anagram.
'not' indicates named letters should be taken away.
'studbook' with 'ok' taken away is 'studbo'.
'studbo' with letters rearranged gives 'DOUBTS'.
'in' acts as a link.
(Other definitions for doubts that I've seen before include "Misgivings" , "Uncertainties" , "Has reservations, is uncertain" , "Isn't certain about, has reservations" , "Suspicions" .)