The most common and most likely answer to this crossword clue is the 5 letter word SMARM
I've seen this crossword clue in The Daily Telegraph
S
M
A
R
M
Buddy explains!

The straight part
We know that "creep" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case creep = smarm.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 SMARM mean?
SMARM noun- Smarmy language or behavior.
- A style of fan fiction in which characters are warm and caring toward each other but without sexual overtones.
- To fawn, to be unctuous.
- To address in a fawning and unctuous manner.