Shelter takes energy, in principle (5)
I believe the answer is:
tenet
'principle' is the definition.
(a tenet is a principle or value)
'shelter takes energy' is the wordplay.
'shelter' becomes 'tent' (a tent is a kind of shelter).
'takes' indicates putting letters inside.
'energy' becomes 'e' (physics symbol as in E=mc2).
'tent' enclosing 'e' is 'TENET'.
'in' is the link.
(Other definitions for tenet that I've seen before include "Conviction" , "Teaching" , "Item of belief" , "A palindromic doctrine or belief" , "you may believe in it" .)