Romeo is ascending on part of flight (5)
I believe the answer is:
riser
'part of flight' is the definition.
(I've seen this before)
'romeo is ascending on' is the wordplay.
'romeo' becomes 'r' (phonetic alphabet: alpha, bravo, charlie etc.).
'ascending' is a reversal indicator.
'on' becomes 're' ('on' can be mean 'with reference to').
're' written backwards gives 'er'.
'r'+'is'+'er'='RISER'
(Other definitions for riser that I've seen before include "Stair front" , "Pipe for upward flow" , "upright portion of step" , "Front of a step; vertical pipe" , "Person getting out of bed" .)