The most common and most likely answer to this crossword clue is the 10 letter word INFILTRATE
I've seen this crossword clue in The Daily Telegraph
I
N
F
I
L
T
R
A
T
E
Buddy explains!

The straight part
We know that "Enter" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case enter = infiltrate.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 INFILTRATE mean?
INFILTRATE noun- Any undesirable substance or group of cells that has made its way into part of the body.
- To surreptitiously penetrate, enter or gain access to.
- (of a liquid) To pass through something by filtration.
- To cause (a liquid) to pass through something by filtration.
- To invade or penetrate a tissue or organ.
- To send (soldiers, spies etc.) through gaps in the enemy line.
- (of an intravenous needle) To move from a vein, remaining in the body.