Substantial article carried in both hands, say, to be returned (5)
I believe the answer is:
large
'substantial' is the definition.
(I've seen this before)
'article carried in both hands say to be returned' is the wordplay.
'article' becomes 'a' (indefinite article).
'carried in' indicates putting letters inside.
'both hands' becomes 'lr' (abbreviations for left and right).
'say' becomes 'eg'.
'to be returned' shows that the letters should be reversed in order (I've seen 'is returned' mean this).
'eg' written backwards gives 'ge'.
'a' inserted within 'lr' is 'lar'.
'lar'+'ge'='LARGE'
(Other definitions for large that I've seen before include "Generous" , "Sizeable" , "Tall" , "Vast" , "Regal (anag)" .)