The most common and most likely answer to this crossword clue is the 3 plus 5 plus 3 plus 2 plus 8 letter word ALL MOUTH AND NO TROUSERS
A
L
L
M
O
U
T
H
A
N
D
N
O
T
R
O
U
S
E
R
S
Buddy explains!
The straight part
We know that "Show" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case show = all mouth and no trousers.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.