It may be proper for a sister to admit nothing (4)
I believe the answer is:
noun
'it may be proper' is the definition.
(I've seen this in another clue)
'sister to admit nothing' is the wordplay.
'sister' becomes 'nun' (sister is a term of address for a nun).
'to admit' indicates putting letters inside.
'nothing' becomes 'O' (looks like zero - 0).
'nun' enclosing 'o' is 'NOUN'.
'for a' acts as a link.
(Other definitions for noun that I've seen before include "Category of word" , "Type of word" , "Naming word (grammar)" , "Word identifying person, place or thing" , "Word used as a name for something or person" .)