Second number scheme on European aircraft (9)
I believe the answer is:
monoplane
'aircraft' is the definition.
(monoplane is a kind of aircraft)
'second number scheme on european' is the wordplay.
'second' becomes 'mo' (both are short periods of time).
'number' becomes 'no' (abbreviation for 'number').
'scheme' becomes 'plan' (I've seen this before).
'on' is a charade indicator (letters next to each other).
'european' becomes 'e' (abbreviation e.g. EU).
'mo'+'no'+'plan'+'e'='MONOPLANE'
(Other definitions for monoplane that I've seen before include "One-winged aircraft" , "Single-winged aircraft" .)