It's sad, Joe being in the cart, perhaps (6)
I believe the answer is:
tragic
'it's sad' is the definition.
(similar in meaning)
'joe being in the cart perhaps' is the wordplay.
'joe' becomes 'GI' (GI Joe).
'being in the' means one lot of letters goes inside another.
'perhaps' is an anagram indicator.
'cart' is an anagram of 'trac'.
'gi' put within 'trac' is 'TRAGIC'.
(Other definitions for tragic that I've seen before include "Involving grief of death or destruction" , "Deeply distressing" , "Unfortunate" , "Calamitous, very sad" , "Terrible" .)