The most common and most likely answer to this crossword clue is the 9 letter word PAINTWORK
I've seen this crossword clue in The Guardian
P
A
I
N
T
W
O
R
K
More clues leading to the result PAINTWORK
Buddy explains!

The straight part
We know that "act of touching up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case act of touching up = paintwork.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 PAINTWORK mean?
PAINTWORK noun- The painted surface of a car, building, etc.