Insubstantial small muscle lacks oxygen (8)
I believe the answer is:
spectral
'insubstantial' is the definition.
(like a ghost)
'small muscle lacks oxygen' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'muscle' becomes 'pectoral' (pectoral muscle is a kind of muscle).
'lacks' suggests deleting specific letters.
'oxygen' becomes 'o' (atomic symbol for oxygen).
'pectoral' with 'o' taken away is 'pectral'.
's'+'pectral'='SPECTRAL'
(Other definitions for spectral that I've seen before include "Like a colourful band" , "Spooky" , "Of or like a ghost" , "Rap Celts for being ghostly" , "Ghastly" .)