It's clear it sounds like an aircraft (5)
I believe the answer is:
plain
'it's clear' is the definition.
(synonyms)
'it sounds like an aircraft' is the wordplay.
'it sounds like' indicates a 'sounds like' (homophone) clue.
'an aircraft' becomes 'plane' (plane is a kind of aircraft).
'plane' is a homophone of 'PLAIN'.
(Other definitions for plain that I've seen before include "Unadorned; evident" , "Unadorned, not elaborate" , "Unembellished" , "no oil painting features" , "not striking" .)