Seen in sonar: 'Whale ... another whale' (7)
I believe the answer is:
Here is my best explanation:
'whale' is the definition.
(narwhal is a kind of whale)
'seen in sonar whale another' is the wordplay.
'seen in' indicates the answer is hidden within the clue.
'NARWHAL' is hidden in the letters of 'sonar whale another'.