Put on too much weight (8)
'put on too much weight' is the definition.
(I've seen this before)
'on too much weight' is the wordplay.
'on' says to put letters next to each other.
'too much' becomes 'over' (I've seen this in other clues).
'weight' becomes 'load' (load is a kind of weight).
'over' next to 'load' is 'OVERLOAD'.
(Other definitions for overload that I've seen before include "Fill too full" , "Put on too much weight" , "Put too great a demand on (a system)" , "Burden excessively" , "saddle" .)
(Other definitions for overburden that I've seen before include "demand too much work" , "Worry too much" , "Load excessively" .)
(Another definition for overloaded that I've seen is " Carrying too much".)