A sisterly surname (5)
I believe the answer is:
nolan
'surname' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'a sisterly' is the wordplay.
I cannot quite see how this works, but
'a' could be 'an' and 'an' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for nolan that I've seen before include "Film director - singing sisters" , "Sir Sidney - - , Australian painter" .)