Adolescent cheers up dons put in prison (8)
I believe the answer is:
immature
'adolescent' is the definition.
(similar in meaning)
'cheers up dons put in prison' is the wordplay.
'cheers' becomes 'ta' (both can informally mean thank you).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'dons' is an insertion indicator (don means to wear or put on).
'put in prison' becomes 'immure' (to immure is to confine or imprison somewhere).
'ta' written backwards gives 'at'.
'at' inserted within 'immure' is 'IMMATURE'.
(Other definitions for immature that I've seen before include "Not fully ripe or developed" , "Not yet adult or fully grown" , "Like childish behaviour" , "Underdeveloped - childish" , "Lacking in development" .)