Crazy write-up, mostly on Eastern coral (9)
I believe the answer is:
madrepore
'coral' is the definition.
(I know that madrepore is a type of coral)
'crazy write-up mostly on eastern' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'write-up' becomes 'report' (I've seen this before).
'mostly' means to remove the last letter (most but not all of the word).
'on' means one lot of letters go next to another.
'eastern' becomes 'e'.
'report' with its last letter taken away is 'repor'.
'mad'+'repor'+'e'='MADREPORE'
(Another definition for madrepore that I've seen is " variety of coral".)