The most common and most likely answer to this crossword clue is the 7 letter word RASTERS
I've seen this crossword clue in Financial Times
R
A
S
T
E
R
S
Buddy explains!

The straight part
We know that "lines on television" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case lines on television = rasters.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 RASTERS mean?
RASTERS noun- A scanning pattern of parallel lines that form the display of an image projected on a cathode-ray tube of a television set or display screen.
- A bitmap image, consisting of a grid of pixels, stored as a sequence of lines.