The most common and most likely answer to this crossword clue is the 7 letter word MUDDIER
I've seen this crossword clue in The Daily Telegraph
M
U
D
D
I
E
R
More clues leading to the result MUDDIER
Buddy explains!

The straight part
We know that "dirtier" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case dirtier = muddier.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 MUDDIER mean?
MUDDIER adjective- Covered with or full of mud or wet soil.
- With mud or other sediment brought into suspension, turbid.
- Not clear; mixed up or blurry.
- Confused; stupid; incoherent; vague.
- Soiled with feces.
- One who muddies or obscures something.