Orca trained to learn first hymn (5)
I believe the answer is:
carol
'hymn' is the definition.
(thesaurus)
'orca trained to learn first' is the wordplay.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'to' is a charade indicator (letters next to each other).
'first' suggests taking the first letters.
The initial letter of 'learn' is 'l'.
'orca' anagrammed gives 'caro'.
'caro'+'l'='CAROL'
(Other definitions for carol that I've seen before include "Joyful hymn" , "and 27: Waits" , "Joyful song - woman's name" , "sing happily" , "Girl" .)