Ornamental tail of large fabulous bird turned into case (10)

'ornamental' is the definition.
(synonyms)

'tail of large fabulous bird turned into case' is the wordplay.
'tail of' suggests the final letters.
'fabulous bird' becomes 'roc' (mythological bird of prey).
'turned' is a reversal indicator.
'into' is an insertion indicator.
'case' becomes 'dative' (the dative case in grammar).
The final letter of 'large' is 'e'.
'roc' back-to-front is 'cor'.
'e'+'cor'='ecor'
'ecor' put inside 'dative' is 'DECORATIVE'.