Reversing wagon over Joe; how distressing (6)
I believe the answer is:
tragic
'distressing' is the definition.
(I've seen this before)
'reversing wagon over joe' is the wordplay.
'reversing' shows that the letters should be reversed in order.
'wagon' becomes 'cart' (cart is a kind of wagon).
'over' means one lot of letters goes inside another.
'joe' becomes 'GI' (GI Joe).
'cart' in reverse letter order is 'trac'.
'trac' enclosing 'gi' is 'TRAGIC'.
'how' acts as a link.
(Other definitions for tragic that I've seen before include "Exceedingly sad" , "Intensely sad and distressing" , "'Very, very sad (6)'" , "Extremely sad; Mahler's 6th nickname" , "Disastrous, appalling" .)