The most common and most likely answer to this crossword clue is the 7 letter word STERNLY
I've seen this crossword clue in The Guardian
S
T
E
R
N
L
Y
More clues leading to the result STERNLY
- Relentlessly back Liberal youth leader
- Rent out foxy clothes in serious fashion
- Like fox consuming bird without pity?
- Description of a fox eating bird in a grim manner
- Seriously back heartless lady
- How the man at the back speaks?
- Seriously bad style worn by sailors
- Seriously clever, catching seabird
- Try Len's stew in a not-so-nice manner
Buddy explains!

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 STERNLY mean?
STERNLY adverb- In a stern manner.