Level of interest bishop's shown over a frightful teaser (4,4)
I believe the answer is:
base rate
'level of interest' is the definition.
(I've seen this before)
'bishop's shown over a frightful teaser' is the wordplay.
'bishop' becomes 'b' (abbreviation used in chess).
'shown over' is a charade indicator (letters next to each other) (in a down clue, some letters go above others).
'frightful' is an anagram indicator (I've seen this in other clues).
'teaser' with letters rearranged gives 'serate'.
'b'+'a'+'serate'='BASE RATE'
(Other definitions for base rate that I've seen before include "official interest" , "Bank of England lending index" , "Interest charged by the Bank of England to commercial banks" , "Interest level set by the Bank of England" .)