The most common and most likely answer to this crossword clue is the 11 plus 13 letter word ENLARGEMENT (GREENMANTLE)
E
N
L
A
R
G
E
M
E
N
T
(
G
R
E
E
N
M
A
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T
L
E
)
Buddy explains!
The straight part
We know that "increase in size" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case increase in size = enlargement (greenmantle).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.