The most common and most likely answer to this crossword clue is the 7 letter word REMIBOT
R
E
M
I
B
O
T
Buddy explains!
The straight part
We know that "R/eplaced X-ray unit, Nigerian, on time" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case r/eplaced x-ray unit, nigerian, on time = remibot.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.