The most common and most likely answer to this crossword clue is the 8 letter word STURDIER
I've seen this crossword clue in The Daily Telegraph
S
T
U
R
D
I
E
R
More clues leading to the result STURDIER
- More substantial runs between lessons right for son in the end
- More brawny student getting to grips with arithmetic, say?
- More robust undergraduate stifles resistance
- One learning about origin of reform is more resolute
- More resolute learner comprehending arithmetic?
- Student consuming last of lager and more stout
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 STURDIER mean?
STURDIER adjective- Of firm build; stiff; stout; strong.
- Solid in structure or person.
- Foolishly obstinate or resolute; stubborn.
- Resolute, in a good sense; or firm, unyielding quality.