Substantial account correct, yet having occasional omissions (8)
I believe the answer is:
concrete
'substantial' is the definition.
(real, having substance)
'account correct yet having occasional omissions' is the wordplay.
'having occasional omissions' means one should take alternating letters (letters removed at regular intervals or occasions).
'account'+'correct'+'yet'='accountcorrectyet'
The alternate letters of 'accountcorrectyet' are 'CONCRETE'.
(Other definitions for concrete that I've seen before include "Building material is not abstract" , "Substantial, real" , "It's composed of sand, cement and water" , "Solid, real" , "Firm" .)