The most common and most likely answer to this crossword clue is the 7 letter word VISCOUS
I've seen this crossword clue in The Daily Telegraph
V
I
S
C
O
U
S
Buddy explains!

The straight part
We know that "Tacky" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tacky = viscous.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 VISCOUS mean?
VISCOUS adjective- Having a thick, sticky consistency between solid and liquid.
- Of or pertaining to viscosity.