The most common and most likely answer to this crossword clue is the 11 plus 4 letter word CONVENIENCE FOOD
I've seen this crossword clue in The Daily Telegraph
C
O
N
V
E
N
I
E
N
C
E
F
O
O
D
More clues leading to the result CONVENIENCE FOOD
Buddy explains!
The straight part
We know that "ready-made grub" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ready-made grub = convenience food.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.