True capital of Moroccan kingdom (5)
I believe the answer is:
realm
'kingdom' is the definition.
(thesaurus)
'true capital of moroccan' is the wordplay.
'true' becomes 'real' ('real' can be similar in meaning to 'true').
'capital of' says to take the initial letters.
The initial letter of 'moroccan' is 'm'.
'real'+'m'='REALM'
(Other definitions for realm that I've seen before include "Kingdom; field of interest" , "Lamer (anag.)" , "Monarchy" , "monarch's responsibility" , "Area of knowledge under discussion" .)