The most common and most likely answer to this crossword clue is the 9 letter word REPLENISH
R
E
P
L
E
N
I
S
H
More clues leading to the result REPLENISH
- Retailer, quiet about poor line, is to stock up again
- Wrong interrupting supporter turning to reload
- Assistant nurses behave badly, coming back for refill
- Help rinse new stock
- About Dick imbibing last of alcohol and heroin – top up
- Top up liquid left in sphere
- Assistant admits guilt, returns make up
- Pilsner? He ordered refill
- Helpers in mess supply seconds
- Top up helpers in cast
Buddy explains!

The straight part
We know that "top-up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case top-up = replenish.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 REPLENISH mean?
REPLENISH verb- To refill; to renew; to supply again or to add a fresh quantity to.
- To fill up; to complete; to supply fully.
- To finish; to complete; to perfect.