The most common and most likely answer to this crossword clue is the 6 plus 7 letter word BORDER CONTROL
I've seen this crossword clue in Financial Times
B
O
R
D
E
R
C
O
N
T
R
O
L
Buddy explains!
The straight part
We know that "a key concern of Ukip" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a key concern of ukip = border control.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.