Corrupt jockey keeps at it, somehow (7)
I believe the answer is:
vitiate
'corrupt' is the definition.
(I know that corrupt can be written as vitiate)
'jockey keeps at it somehow' is the wordplay.
'jockey' becomes 'vie' (I've seen this in another clue).
'keeps' means one lot of letters goes inside another (inserted letters are kept inside).
'somehow' indicates an anagram.
'at'+'it'='atit'
'atit' anagrammed gives 'tiat'.
'vie' going around 'tiat' is 'VITIATE'.
(Other definitions for vitiate that I've seen before include "ruin" , "Debase - render defective" , "Corrupt" .)