Youth’s mainly young and small (5)
I believe the answer is:
teeny
'small' is the definition.
(similar in meaning)
'youth's mainly young' is the wordplay.
'youth's' becomes 'teens'.
'mainly' means to remove the last letter.
'young' becomes 'y'.
'teens' with its last letter taken away is 'teen'.
'teen'+'y'='TEENY'
'and' is the link.
(Other definitions for teeny that I've seen before include "Extremely small" , "Tiny" , "Very little relating" , "Minuscule" , "Very small (informal)" .)