The most common and most likely answer to this crossword clue is the 12 letter word UNDERSCORING
I've seen this crossword clue in The Daily Telegraph
U
N
D
E
R
S
C
O
R
I
N
G
More clues leading to the result UNDERSCORING
Buddy explains!

The straight part
We know that "Putting emphasis on" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case putting emphasis on = underscoring.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.
Crossword Answer definitions
UNDERSCORING verb- To underline; to mark a line beneath text.
- To emphasize or draw attention to.
- An underline.