The heartless bag principle (5)
I believe the answer is:
tenet
'principle' is the definition.
(I've seen this before)
'the heartless bag' is the wordplay.
'heartless' suggests removing the centre.
'bag' becomes 'net' (I've seen this in other clues).
'the' with its middle removed is 'te'.
'te'+'net'='TENET'
(Other definitions for tenet that I've seen before include "A belief or creed - a palindrome" , "A palindromic doctrine or belief" , "Belief held as true" , "Canon" , "Dogma or principle" .)