The most common and most likely answer to this crossword clue is the 14 letter word UNMENTIONABLES
I've seen this crossword clue in The Times
U
N
M
E
N
T
I
O
N
A
B
L
E
S
Buddy explains!
The straight part
We know that "Underwear for mum?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case underwear for mum? = unmentionables.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 UNMENTIONABLES mean?
UNMENTIONABLES noun- Something not to be discussed in polite society.
- Undergarments, underwear, drawers
- Genitals
- Breeches, trousers