The most common and most likely answer to this crossword clue is the 7 letter word NAPPIES
I've seen this crossword clue in The Guardian
N
A
P
P
I
E
S
Buddy explains!
The straight part
We know that "dirty ones need changing" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dirty ones need changing = nappies.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 NAPPIES mean?
NAPPIES noun- An absorbent garment worn by a baby who does not yet have voluntary control of his or her bladder and bowels or by someone who is incontinent; a diaper.
- To put a nappy on.
- A shallow, flat-bottomed earthenware or glass bowl with sloping sides.