Some loathe or yearn for a system of ideas (6)
I believe the answer is:
theory
'a system of ideas' is the definition.
(a theory is a system of ideas)
'some loathe or yearn' is the wordplay.
'some' says the answer is hidden in the clue (some of the parts of the word are used).
'THEORY' can be found hidden inside 'loathe or yearn'.
'for' is the link.
(Other definitions for theory that I've seen before include "Scientific conjecture" , "Supposition intended to explain observations" , "Suggested explanation" , "Opinion or supposition to explain something" , "Principle on which practice is based" .)