Adolescent youth's head is minuscule (5)
I believe the answer is:
teeny
'minuscule' is the definition.
(I've seen this before)
'adolescent youth's head' is the wordplay.
'adolescent' becomes 'teen' (I've seen this before).
'head' indicates taking the first letters.
The first letter of 'youths' is 'y'.
'teen'+'y'='TEENY'
'is' acts as a link.
(Other definitions for teeny that I've seen before include "Wee" , "Extremely small" , "Very small (informal)" , "Informally very small" , "Minuscule" .)