Flatfoot beginning to compromise operation (3)
I believe the answer is:
cop
'flatfoot' is the definition.
(I've seen this in another clue)
'beginning to compromise operation' is the wordplay.
'beginning to' indicates taking the first letters.
'operation' becomes 'op'.
The initial letter of 'compromise' is 'c'.
'c'+'op'='COP'
(Other definitions for cop that I've seen before include "Policeman (slang)" , "Short policeman" , "Member of the police force and an arrest" , "Policeman (colloq.)" , "Police officer (slang)" .)