Rex splits us up to seize control (5)
I believe the answer is:
usurp
'seize control' is the definition.
'usurp' can be an answer for 'seize' (I've seen this before). I'm not certain of the 'control' bit.
'rex splits us up' is the wordplay.
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'splits' indicates putting letters inside (inserted letters split open the word).
'us'+'up'='usup'
'r' placed into 'usup' is 'USURP'.
'to' is the link.
Can you help me to learn more?
(Other definitions for usurp that I've seen before include "Take power unlawfully" , "Take possession of, illegally" , "Take control of without authority" , "Supplant (someone in power)" , "Assume authority" .)