The most common and most likely answer to this crossword clue is the 4 letter word SEAL
I've seen this crossword clue in The Daily Telegraph
S
E
A
L
More clues leading to the result SEAL
Buddy explains!

The straight part
We know that "secure" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case secure = seal.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 SEAL mean?
SEAL noun- A pinniped (Pinnipedia), particularly an earless seal (true seal) or eared seal.
- A bearing representing a creature something like a walrus.
- To hunt seals.
- A stamp used to impress a design on a soft substance such as wax.
- An impression of such stamp on wax, paper or other material used for sealing.
- A design or insignia usually associated with an organization or an official role.
- Anything that secures or authenticates.
- Something which will be visibly damaged if a covering or container is opened, and which may or may not bear an official design.
- Confirmation or approval, or an indication of this.
- Something designed to prevent liquids or gases from leaking through a joint.
- A tight closure, secure against leakage.
- A chakra.
- To place a seal on (a document).
- To mark with a stamp, as an evidence of standard exactness, legal size, or merchantable quality.
- To fasten (something) so that it cannot be opened without visible damage.
- To prevent people or vehicles from crossing (something).
- To close securely to prevent leakage.
- To place in a sealed container.
- To place a notation of one's next move in a sealed envelope to be opened after an adjournment.
- To guarantee.
- To fix, as a piece of iron in a wall, with cement or plaster, etc.
- To close by means of a seal.
- To confirm or set apart as a second or additional wife.
- To tie up animals (especially cattle) in their stalls.