Trilby’s first in hat box for this? (5)
I believe the answer is:
title
'box for this?' is the definition.
The definition suggests a singular noun which matches the answer.
'trilby's first in hat' is the wordplay.
'trilby's first' becomes 't' (1st letter of 'trilby').
'in' is an insertion indicator.
'hat' becomes 'tile' (old-fashioned term for a hat).
't' placed within 'tile' is 'TITLE'.
Can you help me to learn more?
(Other definitions for title that I've seen before include "label" , "Designation of honour, rank or office" , "'The name of a book, say (5)'" , "Book's name" , "Chapter heading" .)