Letter backed old king invading country (8)
I believe the answer is:
landlord
'letter' is the definition.
(I've seen this before)
'backed old king invading country' is the wordplay.
'backed' says the letters should be written backwards.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'invading' means one lot of letters goes inside another.
'country' becomes 'land' (synonyms).
'old' written backwards gives 'dlo'.
'dlo'+'r'='dlor'
'dlor' inserted into 'land' is 'LANDLORD'.
(Other definitions for landlord that I've seen before include "He lets out building for rent" , "He rents land or premises" , "Male lessor" , "Pub proprietor" , "One who rents out property" .)