PM's predecessor said to be lacking principles (6)
I believe the answer is:
amoral
'lacking principles' is the definition.
(I've seen this in another clue)
'pm's predecessor said to be' is the wordplay.
I cannot quite understand how this works, but
'said' could be 'oral' (both can mean spoken) and 'oral' is found within the answer.
'be' could be 'am' and 'am' is present in 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" .)