The most common and most likely answer to this crossword clue is the 5 letter word CURBS
I've seen this crossword clue in The Daily Telegraph
C
U
R
B
S
Buddy explains!
The straight part
We know that "Puts a block on" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case puts a block on = curbs.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 CURBS mean?
CURBS noun- A concrete margin along the edge of a road; a kerb (UK, Australia, New Zealand)
- A raised margin along the edge of something, such as a well or the eye of a dome, as a strengthening.
- Something that checks or restrains; a restraint.
- A riding or driving bit for a horse that has rein action which amplifies the pressure in the mouth by leverage advantage placing pressure on the poll via the crown piece of the bridle and chin groove via a curb chain.
- A sidewalk, covered or partially enclosed, bordering the airport terminal road system with adjacent paved areas to permit vehicles to off-load or load passengers.
- A swelling on the back part of the hind leg of a horse, just behind the lowest part of the hock joint, generally causing lameness.