This cart carries heavy load three feet up (4)
I believe the answer is:
dray
'this cart carries heavy load' is the definition.
'dray' can be an answer for 'cart' (a cart without sides for delivering heavy loads). I am not sure about the remainder of the definition.
'three feet up' is the wordplay.
'three feet' becomes 'yard' (I've seen this before).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'yard' back-to-front is 'DRAY'.
Can you help me to learn more?
(Other definitions for dray that I've seen before include "Cart without sides, for delivering barrels" , "Low truck" , "Flat-bed truck" , "Cart used for heavy loads" , "Brewer's horse-drawn cart" .)