Rex wrecked us in wrecks (5)
I believe the answer is:
ruins
'wrecks' is the definition.
(I've seen this before)
'rex wrecked us in' is the wordplay.
'rex' becomes 'R' (abbreviation e.g. GR=George Rex, King George).
'wrecked' indicates an anagram.
'us'+'in'='usin'
'usin' anagrammed gives 'uins'.
'r'+'uins'='RUINS'
(Other definitions for ruins that I've seen before include "Spoils" , "Damages irreparably" , "Damages beyond repair" , "Remains to be seen" , "dilapidated buildings" .)