Republican is meeting HM? That’s certainly a step up! (5)
I believe the answer is:
riser
'a step up' is the definition.
(I've seen this before)
'republican is meeting hm?' is the wordplay.
'republican' becomes 'r' (abbreviation).
'meeting' is a charade indicator (letters next to each other).
'hm?' becomes 'er' (HM Her Majesty, ER Elizabeth Regina).
'r'+'is'+'er'='RISER'
'that's certainly' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for riser that I've seen before include "Upright part of stair step" , "Section of flight" , "Upright part of step - vertical pipe" , "Front of a step; vertical pipe" , "A bit of a flight" .)