Insubstantial part of growth industry (4)
I believe the answer is:
thin
'insubstantial' is the definition.
(I've seen this in another clue)
'part of growth industry' is the wordplay.
'part of' says the answer is hidden in the clue.
'THIN' can be found hidden inside 'growth industry'.
(Other definitions for thin that I've seen before include "Narrow, slim" , "Meagre" , "With little thickness" , "Lacking substance; not dense" , "Dilute; lean" .)