A lump I have is substantial (7)
I believe the answer is:
massive
'substantial' is the definition.
(thesaurus)
'a lump i have' is the wordplay.
'a lump' becomes 'mass' (I have seen 'Lump of material' mean 'mass' so perhaps 'lump' could also mean 'mass').
'i have' becomes 'I've' (contraction of 'I have').
'mass'+'ive'='MASSIVE'
'is' acts as a link.
(Other definitions for massive that I've seen before include "Bulky, weighty" , "Exceptionally large" , "Great" , "Colossal" , "Not wavering" .)