Support includes a quiet youngster (7)
I believe the answer is:
sapling
'youngster' is the definition.
(thesaurus)
'support includes a quiet' is the wordplay.
'support' becomes 'sling' (sling is a type of support for member (ie an arm)).
'includes' is an insertion indicator.
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'a'+'p'='ap'
'sling' going around 'ap' is 'SAPLING'.
(Other definitions for sapling that I've seen before include "Poetic youth" , "Baby tree" , "small bit of wood" , "Little tree" , "Young slender tree" .)