Substantial magazine piece: 'Put on weight' (6)
I believe the answer is:
spread
I believe this is a double definition.
'substantial magazine piece' is the first definition.
(eg a full-page spread)
'put on weight' is the second definition.
(spread can mean to gain weight)
(Other definitions for spread that I've seen before include "Unfold - scatter" , "Disseminate through padres" , "Scatter or distribute more thinly" , "Distribute or disperse widely" , "expansion?" .)