Boastful European got First in chemistry — awful loudmouth initially (11)
I believe the answer is:
egotistical
'boastful' is the definition.
(thesaurus)
'european got first in chemistry awful loudmouth initially' is the wordplay.
'european' becomes 'e' (abbreviation e.g. EU).
'first in' becomes 'isti' (I can't explain this - if you can you should give a lot more credence to this answer).
'initially' suggests taking the first letters.
The first letters of 'chemistry awful loudmouth' is 'cal'.
'e'+'got'+'isti'+'cal'='EGOTISTICAL'
(Other definitions for egotistical that I've seen before include "Conceited and boastful" , "Highly self-absorbed" , "Excessively conceited" , "Selfish" , "Self-important" .)