Spontaneous rascal lives differently after university (9)
I believe the answer is:
impulsive
'spontaneous' is the definition.
(similar in meaning)
'rascal lives differently after university' is the wordplay.
'rascal' becomes 'imp' (synonyms).
'differently' indicates anagramming the letters.
'after' says to put letters next to each other.
'university' becomes 'u' (abbreviation for university).
'lives' anagrammed gives 'lsive'.
'lsive' put after 'u' is 'ulsive'.
'imp'+'ulsive'='IMPULSIVE'
(Other definitions for impulsive that I've seen before include "Hot-headed" , "responding promptly" , "Spontaneous" , "Done without forethought" , "Rash" .)