Penny puts on weight in spring (6)
I believe the answer is:
pounce
'spring' is the definition.
(I know that pounce is a type of spring)
'penny puts on weight' is the wordplay.
'penny' becomes 'p' (short for pence).
'puts on' means one lot of letters go next to another.
'weight' becomes 'ounce' (unit of weight).
'p'+'ounce'='POUNCE'
'in' acts as a link.
(Other definitions for pounce that I've seen before include "Spring upon suddenly" , "Jump or spring down as on prey" , "Swoop down suddenly" , "Spring down on and grasp" , "Suddenly spring; fine powder" .)