The most common and most likely answer to this crossword clue is the 4 letter word GUCK
G
U
C
K
Buddy explains!

The straight part
We know that "certainly won’t appeal!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case certainly won’t appeal! = guck.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.
What does GUCK mean?
GUCK noun- The hybrid offspring of a goose and a duck
- An unpleasant sticky substance; goo, gunk.