Infantile learner withdrawn from cool course (8)
I believe the answer is:
childish
'infantile' is the definition.
('childish' can be a synonym of 'infantile')
'learner withdrawn from cool course' is the wordplay.
'learner' becomes 'l'.
'withdrawn from' is a deletion indicator.
'cool' becomes 'chill' ('chill' can be a synonym of 'cool').
'course' becomes 'dish' (I've seen this before**).
'chill' with 'l' taken out is 'chil'.
'chil'+'dish'='CHILDISH'
(Other definitions for childish that I've seen before include "Puerile" , "Juvenile" , "Silly, immature" .)