Utterly absurd new supporter, so excited initially (12)
I believe the answer is:
preposterous
'utterly absurd' is the definition.
(I've seen this before)
'new supporter so excited initially' is the wordplay.
'new' indicates an anagram.
'initially' suggests taking the first letters.
The initial letter of 'excited' is 'e'.
'supporter'+'so'+'e'='supportersoe'
'supportersoe' with letters rearranged gives 'PREPOSTEROUS'.
(Other definitions for preposterous that I've seen before include "Crazy" , "Quite absurd" , "Ludicrous" , "Utterly ridiculous" , "Utterly absurd" .)