Moral principles come in one thickness (5)
I believe the answer is:
ethic
'moral principles' is the definition.
(I've seen this before)
'come in one thickness' is the wordplay.
'come in' indicates the answer is hidden within the clue (I've seen 'coming in' mean this).
'ETHIC' is hidden within 'one thickness'.
(Other definitions for ethic that I've seen before include "Principles accepted by a social group" , "type of behaviour" , "Set of moral principles" , "Ideology" , "Set of moral values" .)