Criminal gang caught by Iron Curtain border? (6)
I believe the answer is:
fringe
'curtain border?' is the definition.
(fringe could refer to the tassels on a curtain)
'criminal gang caught by iron' is the wordplay.
'criminal gang' becomes 'ring' (eg a drugs ring).
'caught by' indicates putting letters inside.
'iron' becomes 'fe' (Fe is the chemical symbol for iron).
'ring' placed into 'fe' is 'FRINGE'.
(Other definitions for fringe that I've seen before include "Hair hanging over the forehead" , "(Decorative) border" , "These benefits are additional to pay" , "Rim" , "Tasselled edging" .)