Flex performing as barrier (6)
I believe the answer is:
cordon
'barrier' is the definition.
(I've seen this before)
'flex performing' is the wordplay.
'flex' becomes 'cord' (I've seen this before).
'performing' becomes 'on' (as in being on stage).
'cord'+'on'='CORDON'
'as' acts as a link.
(Other definitions for cordon that I've seen before include "Ring of people guarding something" , "Line of guards; rope" , "Excellence in cooking is . . . . . . bleu" , "Ribbon" , "Circle of policemen" .)