PM's predecessor said to be lacking principles (6)
I believe the answer is:
amoral
'lacking principles' is the definition.
(I've seen this before)
'pm's predecessor said to be' is the wordplay.
I cannot really understand how this works, but
'said' could be 'oral' (both can mean spoken) and 'oral' is located in the answer.
'be' could be 'am' and 'am' is found within the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for amoral that I've seen before include "Blind to right and wrong" , "Ignoring ethical considerations" , "Lacking any ethical sense" , "Unethical" , "unconcerned about ethics" .)