The most common and most likely answer to this crossword clue is the 5 letter word CRUST
I've seen this crossword clue in The Daily Telegraph
C
R
U
S
T
More clues leading to the result CRUST
Buddy explains!

The straight part
We know that "Outer surface" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case outer surface = crust.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 CRUST mean?
CRUST noun- A more solid, dense or hard layer on a surface or boundary.
- The external, hardened layer of certain foodstuffs, including most types of bread, fried meat, etc.
- An outer layer composed of pastry
- The bread-like base of a pizza.
- The outermost layer of the lithosphere of the Earth.
- The shell of crabs, lobsters, etc.
- Nerve, gall.
- A living.
- To cover with a crust.
- To form a crust.
- A subgenre of punk rock, derived from hardcore punk with extreme metal elements, often with pessimistic lyrics about political and social ills.