What traditionally was carried up the hill -- light when delivered! (4)
I believe the answer is:
pail
'what traditionally was carried up the hill' is the definition.
(as in Jack and Jill, a pail of water)
'light when delivered' is the wordplay.
'light' becomes 'pale' (similar in meaning).
'when delivered' indicates a 'sounds like' (homophone) clue (as the word might sound if delivered out loud).
'pale' is a homophone of 'PAIL'.
(Other definitions for pail that I've seen before include "Open-topped container" , "water-carrier" , "it may go down well" , "Milkmaid's bucket?" , "A bucket as for water" .)