Follow Rex inside to make certain (6)
I believe the answer is:
ensure
'make certain' is the definition.
('ensure' can be a synonym of 'make certain')
'follow rex inside' is the wordplay.
'follow' becomes 'ensue' (I've seen this before).
'rex' becomes 'r' (common abbreviation e.g. GR=George Rex, King Georg).
'inside' is an insertion indicator.
'ensue' going around 'r' is 'ENSURE'.
'to' acts as a link.
(Other definitions for ensure that I've seen before include "See run (anag)" , "Make certain for the future" , "Guarantee" , "Protect" , "Make certain, confirm" .)