Unknown speaker losing head suggests immaturity (5)
I believe the answer is:
youth
'immaturity' is the definition.
(I've seen this before)
'unknown speaker losing head' is the wordplay.
'unknown' becomes 'y' (algebra).
'speaker' becomes 'mouth' (you speak with your mouth).
'losing head' means to remove the first letter.
'mouth' with its initial letter removed is 'outh'.
'y'+'outh'='YOUTH'
'suggests' acts as a link.
(Other definitions for youth that I've seen before include "period before getting old" , "Adolescent" , "Juvenile" , "Youngster" , "Young people" .)