The most common and most likely answer to this crossword clue is the 4 plus 4 letter word SEER FISH
S
E
E
R
F
I
S
H
Buddy explains!
The straight part
We know that "specimen of/mackerel" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case specimen of/mackerel = seer fish.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.