Youngster’s casual shirt with name (4)
I believe the answer is:
teen
'youngster's' is the definition.
(I've seen this before)
'casual shirt with name' is the wordplay.
'casual shirt' becomes 'tee' (I've seen this in another clue).
'with' says to put letters next to each other.
'name' becomes 'n' (common abbreviation e.g. on forms).
'tee'+'n'='TEEN'
(Other definitions for teen that I've seen before include "Youth" , "Juvenile" , "Of 13 to 19 year olds" , "Youngster of a certain age" , "Young person" .)