Human right breached by leader in theory (6)
I believe the answer is:
mortal
'human' is the definition.
(synonyms)
'right breached by leader in theory' is the wordplay.
'right' becomes 'moral' (similar in meaning).
'breached by' indicates putting letters inside (inserted letters break into or breach the word).
'leader in theory' becomes 't' (I can't explain this - if you can you should give a lot more credence to this answer).
'moral' placed around 't' is 'MORTAL'.
Can you help me to learn more?
(Other definitions for mortal that I've seen before include "liable to die" , "Causing death" , "Certain to die" , "Subject to death, like all mankind" , "Very drunk < person" .)