The most common and most likely answer to this crossword clue is the 6 letter word HIJACK
I've seen this crossword clue in Financial Times
H
I
J
A
C
K
More clues leading to the result HIJACK
- Seize greeting card
- Take over a century to separate foursome following the leader of government
- Seize four consecutive letters about account
- Seize control of countryman bypassing European agreement
- Roughly appropriate greeting to a sailor
- Make a grab: hello, sailor!
- “Hello, sailor” is appropriate, when moving
- Take over something at the top of the flagpole, say?
- Call boy’s takeover
- What might be appropriate in the air?
Buddy explains!

The straight part
We know that "Seize control of" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case seize control of = hijack.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 HIJACK mean?
HIJACK noun- An instance of hijacking; the illegal seizure of a vehicle; a hijacking.
- An instance of a seizure and redirection of a process.
- An amendment which deletes the contents of a bill and inserts entirely new provisions.
- Preflop, the position two before the dealer.
- To forcibly stop and seize control of some vehicle in order to rob it or to reach a destination (especially an airplane, truck or a boat).
- To seize control of some process or resource to achieve a purpose other than its originally intended one.
- To seize control of a networked computer by means of infecting it with a worm or other malware, thereby turning it into a zombie.
- To change software settings without a user's knowledge so as to force that user to visit a certain web site (to hijack a browser).
- To introduce an amendment deleting the contents of a bill and inserting entirely new provisions.