The most common and most likely answer to this crossword clue is the 4 letter word LIME
I've seen this crossword clue in Independent
L
I
M
E
More clues leading to the result LIME
- Linden found touching in story
- Greeny-yellow mucus after wiping top of sleeve
- Fruit in slippery gunk, initially wiped
- Sticky stuff for catching birds in tree
- Adam finally taken in by falsehood gets fruit
- Story about male constituent of 21
- Unlimited gin punches left Hoskins green
- Unpleasant secretion wipes out second tree
- Fiction about money being something that grows on trees
- Caustic substance in tree
Buddy explains!

The straight part
We know that "something that grows on trees" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case something that grows on trees = lime.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 LIME mean?
LIME noun- Any inorganic material containing calcium, usually calcium oxide (quicklime) or calcium hydroxide (slaked lime).
- Any gluey or adhesive substance; something which traps or captures someone; sometimes a synonym for birdlime.
- To treat with calcium hydroxide or calcium oxide (lime).
- To smear with birdlime.
- To apply limewash.
- A deciduous tree of the genus Tilia, especially Tilia × europaea; the linden tree, or its wood.
- Any of several green citrus fruit, somewhat smaller and sharper-tasting than a lemon.
- Any of the trees that bear limes, especially Key lime, Citrus aurantiifolia.
- A brilliant, sometimes yellowish, green colour associated with the fruits of a lime tree.
- A fan fiction story which contains sexual references, but stops short of full, explicit descriptions of sexual activity (coined by analogy with lemon).
- Containing lime or lime juice.
- Having the aroma or flavor of lime.
- Lime-green.
- To hang out/socialize in an informal, relaxed environment, especially with friends, for example at a party or on the beach.
- A leash.