The most common and most likely answer to this crossword clue is the 4 letter word MASH
M
A
S
H
More clues leading to the result MASH
Buddy explains!

The straight part
We know that "old TV series" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case old tv series = mash.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 MASH mean?
MASH noun- A mass of mixed ingredients reduced to a soft pulpy state by beating or pressure; a mass of anything in a soft pulpy state.
- Ground or bruised malt, or meal of rye, wheat, corn, or other grain (or a mixture of malt and meal) steeped and stirred in hot water for making the wort.
- Mashed potatoes.
- A mixture of meal or bran and water fed to animals.
- A mess; trouble.
- A gun.
- To convert into a mash; to reduce to a soft pulpy state by beating or pressure
- In brewing, to convert (for example malt, or malt and meal) into the mash which makes wort.
- To press down hard (on).
- To press.
- To prepare a cup of tea in a teapot; to brew (tea).
- To act violently.
- A mesh.
- An infatuation, a crush, a fancy
- A dandy, a masher
- The object of one’s affections (either sex)
- To flirt, to make eyes, to make romantic advances