Babyish solver no good (5)
I believe the answer is:
young
'babyish' is the definition.
(thesaurus)
'solver no good' is the wordplay.
'solver' becomes 'you' (the person solving the clue!).
'no good' becomes 'ng' (abbreviation).
'you'+'ng'='YOUNG'
(Other definitions for young that I've seen before include "interviewer of PMs on the radio" , "Newly-developed" , "Mormon leader" , "issue" , "Fry" .)