The most common and most likely answer to this crossword clue is the 2 plus 1 plus 4 plus 2 plus 5 letter word AT A RATE OF KNOTS
I've seen this crossword clue in The Times
A
T
A
R
A
T
E
O
F
K
N
O
T
S
More clues leading to the result AT A RATE OF KNOTS
Buddy explains!
The straight part
We know that "Yet probably not allowing grannies to keep up?" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case yet probably not allowing grannies to keep up? = at a rate of knots.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.