During seminar, whale is a cetacean (7)
I believe the answer is:
narwhal
'a cetacean' is the definition.
(I know that narwhal is a type of whale)
'during seminar whale is' is the wordplay.
'during' indicates the answer is hidden within the clue.
'NARWHAL' can be found hidden inside 'seminar whale is'.
(Other definitions for narwhal that I've seen before include "Small tusked whale" , "Breed of small whale" , "Small Arctic whale" , "Warn Hal there's a whale here" , "swimmer" .)