Substantial part of secular generation (5)
I believe the answer is:
large
'substantial' is the definition.
(substantial can mean large or considerable)
'part of secular generation' is the wordplay.
'part of' indicates a hidden word.
'LARGE' is hidden in the letters of 'secular generation'.
(Other definitions for large that I've seen before include "Big, outsize" , "substantial" , "Bumper" , "Big, almost regal" , "Massive" .)