Flap when lines cover mirror (5)
I believe the answer is:
lapel
'flap' is the definition.
(I know that lapel is a type of lap)
'lines cover mirror' is the wordplay.
'lines' becomes 'll' (an abbreviation for line written twice).
'cover' is an insertion indicator.
'mirror' becomes 'ape' (both can mean to imitate or copy).
'll' enclosing 'ape' is 'LAPEL'.
'when' is the link.
(Other definitions for lapel that I've seen before include "Folded-back part of coat" , "Jacket fold" , "Lower extension of collar on jacket or coat" , "Continuation of a coat collar" , "Turned-back part of coat collar" .)