Insubstantial western rowing crew? Not so (10)
I believe the answer is:
weightless
'insubstantial' is the definition.
(thesaurus)
'western rowing crew? not so' is the wordplay.
'western' becomes 'w'.
'rowing crew?' becomes 'eight' (crew of eight rowers).
'not so' becomes 'less' (eg 'not so big' means 'less big').
'w'+'eight'+'less'='WEIGHTLESS'
(Other definitions for weightless that I've seen before include "Unimportant" , "In zero gravity" , "Floating" , "like people in space station" , "like astronauts?" .)