The most common and most likely answer to this crossword clue is the 7 letter word ONBOARD
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More clues leading to the result ONBOARD
Buddy explains!

The straight part
We know that "Installed in car, say" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case installed in car, say = onboard.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 ONBOARD mean?
ONBOARD verb- To become a part of a group; to incorporate (someone) into a group.
- To begin to use a product or service; to take (someone) on as a new customer of a product or service.
- Carried or used on or in a vehicle or vessel
- (by extension) Being a part of, being included in, participating in
- On or in a vehicle or vessel; aboard; on board.