Simple flier, we hear? (5)
I believe the answer is:
plain
'simple' is the definition.
(associated in meaning)
'flier we hear?' is the wordplay.
'flier' becomes 'plane' (planes fly).
'we hear?' shows a homophone (sound like).
'plane' sounds like 'PLAIN'.
(Other definitions for plain that I've seen before include "lament" , "Simple or apparent" , "Geographical feature" , "Basic" , "Simple, ordinary" .)