Very little to go on, ultimately, in adolescent years (5-5)
I believe the answer is:
teeny-weeny
'very little' is the definition.
(I've seen this before)
'to go on ultimately in adolescent years' is the wordplay.
'to go on' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'ultimately in' becomes 'weeny' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'adolescent' becomes 'teen' (teenager).
'years' becomes 'y' (abbreviation).
'teen'+'y'='teeny'
'weeny' after 'teeny' is 'TEENY-WEENY'.
(Other definitions for teeny-weeny that I've seen before include "Itsy-bitsy" , "Very small" , "Minute" , "Minuscule" .)