Signal where trade insubstantial (7,5)
I believe the answer is:
traffic light
'signal' is the definition.
(traffic light is a kind of signal)
'trade insubstantial' is the wordplay.
'trade' becomes 'traffic' (trafficking is a kind of trading).
'insubstantial' becomes 'light' (I've seen this before).
'traffic'+'light'='TRAFFIC LIGHT'
'where' acts as a link.
(Other definitions for traffic light that I've seen before include "Vehicle flow controller" , "one wants it to be green" , "A shining contribution to road safety" .)