The most common and most likely answer to this crossword clue is the 7 letter word BURGEON
I've seen this crossword clue in The Guardian
B
U
R
G
E
O
N
More clues leading to the result BURGEON
- Caterpillar turning an age to undergo rapid development
- Mushroom good in Bordeaux when filled with egg
- Comestibles served up on time – that’s swell
- Swell getting starter of boiled egg on
- Swell – food’s up before one goes off
- British medic hasn’t started to develop
- Boy’s first impulse regarding snowball
- Encourage support for British boom
- Grow food over very long time
- Boy takes eggs; primary nutrient to grow
Buddy explains!

The straight part
We know that "Swell" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case swell = burgeon.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 BURGEON mean?
BURGEON noun- A bud, sprout, shoot.
- To grow or expand.
- To swell to the point of bursting.
- Of plants, to bloom, bud.