Blue truck flipped on heavy side (4)
I believe the answer is:
navy
'blue' is the definition.
(shade of blue)
'truck flipped on heavy side' is the wordplay.
'truck' becomes 'van' (van is a kind of truck).
'flipped' says the letters should be written backwards.
'on heavy side' becomes 'y' (I can't explain this - if you can you should give a lot more credence to this answer).
'van' reversed gives 'nav'.
'nav'+'y'='NAVY'
Can you help me to learn more?
(Other definitions for navy that I've seen before include "Nation's warships" , "Marine force" , "Colour; fighting force" , "military arm" , "Shade of the marine forces" .)