The most common and most likely answer to this crossword clue is the 2 plus 4 letter word GO SLOW
G
O
S
L
O
W
Buddy explains!
The straight part
We know that "Work to rule" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case work to rule = go slow.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 GO SLOW mean?
GO SLOW noun- A period of time spent in an unusually slow fashion.
- Deliberate slow working as a protest during an industrial dispute.