The most common and most likely answer to this crossword clue is the 11 letter word CLAPPED-OUT
I've seen this crossword clue in The Guardian
C
L
A
P
P
E
D
-
O
U
T
More clues leading to the result CLAPPED-OUT
Buddy explains!
The straight part
We know that "Knackered/as Cook was before last Test innings" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case knackered/as cook was before last test innings = clapped-out.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.