Good impersonators mouthed expressions (5)
I believe the answer is:
gapes
'mouthed expressions' is the definition.
(a gape is an opening of the mouth)
'good impersonators' is the wordplay.
'good' becomes 'g' (abbreviation).
'impersonators' becomes 'apes' (ape can mean a mimic).
'g'+'apes'='GAPES'
(Other definitions for gapes that I've seen before include "Stares at with mouth wide open" , "Yawns" , "goggles" , "seems very surprised" , "Opens mouth wide in surprise" .)