The most common and most likely answer to this crossword clue is the 8 letter word BABYFACE
I've seen this crossword clue in Financial Times
B
A
B
Y
F
A
C
E
Buddy explains!
The straight part
We know that "Comparatively small mug for /? (8)" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case comparatively small mug for /? (8) = babyface.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 BABYFACE mean?
BABYFACE noun- A face, often chubby, resembling that of a baby; a youthful face.
- A person having such a face, especially a young man having a beardless appearance.
- A headlining wrestler with a persona embodying heroic or virtuous traits and who is regarded as a "good guy", especially one who is handsome and well-conditioned.