Following Rex in and made certain (7)
I believe the answer is:
ensured
'made certain' is the definition.
('ensure' can be a synonym of 'make certain')
'following rex in' is the wordplay.
I cannot really see how this works, but
'rex' could be 'r' (abbreviation e.g. GR=George Rex, King George) and 'r' is found in the answer.
The remaining letters 'ensued' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
'and' is the link.
Can you help me to learn more?
(Other definitions for ensured that I've seen before include "Made certain, clinched" , "Determined" , "Made certain for end user" , "Saw to it" , "Make certain of" .)