Emperor beginning to try on new suit (5)
I believe the answer is:
titus
'emperor' is the definition.
(I've seen this before)
'beginning to try on new suit' is the wordplay.
'beginning to' says to take the initial letters.
'on' is an insertion indicator.
'new' indicates an anagram.
The first letter of 'try' is 't'.
'suit' is an anagram of 'tius'.
't' put within 'tius' is 'TITUS'.
(Other definitions for titus that I've seen before include "Groan maybe" , "- Andronicus (Shak.)" , ". . . . . Andronicus, Shakespearean tragedy" , "A Roman emperor; a Pauline addressee" , "identifying Philemon's neighbour" .)