Register concerning gut (7)
I believe the answer is:
enteron
'gut' is the definition.
(I've seen this before)
'register concerning' is the wordplay.
'register' becomes 'enter' (entering is a kind of registering).
'concerning' becomes 'on' (both can mean 'in relation to').
'enter'+'on'='ENTERON'
(Other definitions for enteron that I've seen before include "element of guts" , "Begin; engage in" , "gut" , "Open" .)