The most common and most likely answer to this crossword clue is the 4 plus 5 letter word MILK TRAIN
I've seen this crossword clue in The Times
M
I
L
K
T
R
A
I
N
Buddy explains!
The straight part
We know that "But it’s certainly no “express”" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case but it’s certainly no “express” = milk train.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.