The most common and most likely answer to this crossword clue is the 10 letter word STRENGTHEN
I've seen this crossword clue in The Guardian
S
T
R
E
N
G
T
H
E
N
More clues leading to the result STRENGTHEN
- Buttress with ends of segment touching, and no good, therefore
- Tip of todger trapped in Gents somehow? In that case, support
- Get NHS rent reviewed and grow stronger
- It can be tied then, ie switched, to reinforce
- Might England’s openers consolidate?
- Crooked teeth — gran’s wanting a new brace
- Run inside Gents, bursting — afterwards, rally
- Socialist rabble nagging starts and ends next rally
- Temper half lost about ultimately galling article in news
- Brace for row, with sweetheart ousting one afterwards
Buddy explains!

The straight part
We know that "grow stronger" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case grow stronger = strengthen.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 STRENGTHEN mean?
STRENGTHEN verb- To make strong or stronger; to add strength to; to increase the strength of; to fortify.
- To empower; to give moral strength to; to encourage; to enhearten.
- To augment; to improve; to intensify.
- To grow strong or stronger.