Vamp at heart said to be without principles (6)
I believe the answer is:
amoral
'be without principles' is the definition.
(I've seen this before)
'vamp at heart said' is the wordplay.
'at heart' indicates the central letters.
'said' becomes 'oral' (I've seen this before).
The central letters of 'vamp' are 'am'.
'am'+'oral'='AMORAL'
'to' is the link.
(Other definitions for amoral that I've seen before include "Unscrupulous" , "Not knowing right or wrong" , "regardless of what's right or wrong" , "unconcerned about ethics" , "Having no standards of right or wrong" .)