Back or rump or bits of sow's entrails (7)
I believe the answer is:
Here is my best explanation:
'back' is the definition.
('endorse' can be a synonym of 'back')
'rump or bits of sow's entrails' is the subsidiary indication.
'rump' becomes 'end'.
'bits of' indicates taking the first letters (I've seen 'bit of' mean this).
The initial letters of 'sows entrails' is 'se'.
'or' is the link.
(Other definitions for endorse that I've seen before include "Approve or underwrite", "Sanction -- red nose (anag)", "Champion", "Docket", "Corroborate", "Sign the back of a cheque", "Give backing to", "Say you approve", "Support (with signature)" and "Affirm".)