Eavesdrop on row that's more substantial (8)
I believe the answer is:
heartier
'more substantial' is the definition.
('hearty' can be a synonym of 'substantial')
'eavesdrop on row' is the wordplay.
'eavesdrop on' becomes 'hear' (I've seen this in another clue).
'row' becomes 'tier' (a tier is a row of things).
'hear'+'tier'='HEARTIER'
'that's' acts as a link.
(Other definitions for heartier that I've seen before include "becoming less restrained" , "More enthusiastic" , "More red-blooded - rehear it (anag)" , "more robust" , "more energetic" .)