Seen in sonar: 'Whale ... another whale' (7)
I believe the answer is:
narwhal
'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'.
(Other definitions for narwhal that I've seen before include "Ocean mammal" , "Tusked sea mammal" , "Arctic whale, the male having a long spiral ivory tusk" , "Tusked marine creature" , "aquatic mammal" .)