In Assyria king is related by blood (4)
I believe the answer is:
akin
'related by blood' is the definition.
(I've seen this before)
'in assyria king is' is the wordplay.
'in' indicates a hidden word.
'AKIN' can be found hidden inside 'assyria king is'.
(Other definitions for akin that I've seen before include "Related or similiar" , "Related or similar - as a relative?" , "Similar in quality" , "Similar, like" , "Close" .)