Moral principles break the ice mostly (5)
I believe the answer is:
ethic
'moral principles' is the definition.
(I've seen this before)
'break the ice mostly' is the wordplay.
'break' is an anagram indicator.
'mostly' means to remove the last letter.
'ice' with its last letter removed is 'ic'.
'the' with letters rearranged gives 'eth'.
'eth'+'ic'='ETHIC'
(Other definitions for ethic that I've seen before include "Moral principle" , "type of behaviour" , "System of rules of behaviour" , "Principles accepted by a social group" , "Moral practice" .)