Engine’s sound and very secure on ascent (5)
I believe the answer is:
vroom
'engine's sound' is the definition.
(I know this)
'very secure on ascent' is the wordplay.
'very' becomes 'v' (abbreviation).
'secure' becomes 'moor' (mooring is a kind of securing).
'on' means one lot of letters go next to another.
'ascent' says the letters should be written in reverse (going up in down clue).
'v' after 'moor' is 'moorv'.
'moorv' in reverse letter order is 'VROOM'.
'and' acts as a link.
(Other definitions for vroom that I've seen before include "Fast revving sound" , "Exclamation imitating a revving car" , "(Of engine) make roaring sound (colloq.)" , "roar away" , "loud engine noise" .)