Schoolboys ultimately mischievous and greedy (7)
I believe the answer is:
selfish
'greedy' is the definition.
(greed is selfish)
'schoolboys ultimately mischievous' is the wordplay.
'ultimately' says to take the final letters.
'mischievous' becomes 'elfish' (synonyms).
The last letter of 'schoolboys' is 's'.
's'+'elfish'='SELFISH'
'and' is the link.
(Other definitions for selfish that I've seen before include "Mercenary" , "inconsiderate" , "Egocentric - mean" , "Concerned only with self" , "Putting own interests first" .)