Threaten corrupt grandee? Not at first (8)
I believe the answer is:
endanger
'threaten' is the definition.
(I know that threaten can be written as endanger)
'corrupt grandee? not at first' is the wordplay.
'corrupt' indicates anagramming the letters.
'at first' indicates taking the first letters.
The initial letter of 'not' is 'n'.
'grandee'+'n'='grandeen'
'grandeen' is an anagram of 'ENDANGER'.
(Other definitions for endanger that I've seen before include "Jeopardize" , "Dan Green (anag.)" , "Put in trouble" , "Place into peril, jeopardise" , "To put one at risk" .)