The most common and most likely answer to this crossword clue is the 12 letter word CLEAN-SHAVEN
I've seen this crossword clue in Independent
C
L
E
A
N
-
S
H
A
V
E
N
Buddy explains!
The straight part
We know that "Smooth" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case smooth = clean-shaven.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 CLEAN-SHAVEN mean?
CLEAN-SHAVEN adjective- Having had hair (especially facial hair) shaved off, so that it is no longer visible.