Tom, say, boxes right lug (4)
I believe the answer is:
cart
'lug' is the definition.
(thesaurus)
'tom say boxes right' is the wordplay.
'tom say' becomes 'cat' (tom is a kind of cat).
'boxes' means one lot of letters goes inside another (the outside letters must 'box' inside letters in).
'right' becomes 'r' (common abbreviation).
'cat' enclosing 'r' is 'CART'.
(Other definitions for cart that I've seen before include "Wheelbarrow" , "Hump" , "Heavy open wagon" , "Two-wheeled, drawn vehicle" , "Open wagon (to come after, not before, the horse)" .)