Private instructor transformed trout (5)
I believe the answer is:
tutor
'private instructor' is the definition.
(synonyms)
'transformed trout' is the wordplay.
'transformed' indicates anagramming the letters.
'trout' with letters rearranged gives 'TUTOR'.
(Other definitions for tutor that I've seen before include "School" , "Private or university teacher" , "Teacher (often private)" , "Master" , "Guardian" .)