The most common and most likely answer to this crossword clue is the 5 letter word RISER
I've seen this crossword clue in The Times
R
I
S
E
R
More clues leading to the result RISER
- Part of stair in tower is eroded
- Republican is meeting HM? That’s certainly a step up!
- Romeo lives with woman in palace: it’s a step up
- Pipe up about title being brought up
- One who gets out of bed, upset about teacher's address
- Part of flight back from Azores I reserved
- Republican is beside monarch for part of flight
- Gentleman upset leading lady for part of flight
- Endless danger, tyre half gone from rear, no tread
- Part of step is elevated, initially, between rivers
Buddy explains!

The straight part
We know that "a step up" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case a step up = riser.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 RISER mean?
RISER noun- Someone or something which rises.
- A platform or stand used to lift or elevate something.
- The vertical part of a step on a staircase.
- The main body of a bow.
- A vertical utility conduit, pipe or path between floors of a building for placement of cables (e.g. telephone, networking), or to convey fluids (e.g. gas, water).
- A pipe connecting an individual exhaust port of an internal combustion engine to the muffler, particularly on aircraft.
- A Manx cat with a showable short tail.
- A strip of webbing joining a parachute's harness to the rigging lines.