Thoughts and principles left out (5)
I believe the answer is:
ideas
'thoughts' is the definition.
('idea' can be a synonym of 'thought')
'principles left out' is the wordplay.
'principles' becomes 'ideals' (I've seen this before).
'left' becomes 'l' (common abbreviation).
'out' suggests deleting specific letters.
'ideals' with 'l' taken out is 'IDEAS'.
'and' is the link.
(Other definitions for ideas that I've seen before include "Fancies" , "Brain waves" , "Thoughts, perceptions" , "thinking" , "Thoughts from Sadie" .)