Measure of cloth weaver’s placed under black bags (4-7)
I believe the answer is:
bell-bottoms
'bags' is the definition.
(type of baggy trousers)
'measure of cloth weaver's placed under black' is the wordplay.
'measure of cloth' becomes 'ell' (I've seen this before).
'weaver's' becomes 'Bottom's' (Bottom the Weaver from A Midsummer Night's Dream).
'placed under' means one lot of letters go next to another.
'black' becomes 'b' (as in pencil classifications).
'ell'+'bottoms'='ellbottoms'
'ellbottoms' after 'b' is 'BELL-BOTTOMS'.
(Other definitions for bell-bottoms that I've seen before include "Wide-flared trousers" , "Naval rating's trousers" , "Flared trousers" , "Sailor's trousers" , "Trousers with a flare below the knee" .)