The most common and most likely answer to this crossword clue is the 15 letter word HOLE-AND-CORNER
I've seen this crossword clue in The Guardian
H
O
L
E
-
A
N
D
-
C
O
R
N
E
R
More clues leading to the result HOLE-AND-CORNER
Buddy explains!

The straight part
We know that "Underhand/Underhand" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case underhand/underhand = hole-and-corner.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 HOLE-AND-CORNER mean?
HOLE-AND-CORNER adjective- Conducted in secret; clandestine.