The most common and most likely answer to this crossword clue is the 7 letter word UNBLOCK
I've seen this crossword clue in The Times
U
N
B
L
O
C
K
Buddy explains!

The straight part
We know that "Loosen" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case loosen = unblock.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 UNBLOCK mean?
UNBLOCK verb- To remove or clear a block or obstruction from.
- To free or make available.
- In whist, to throw away a high card so as not to interrupt one's partner's long suit.