It's very serious when Joe's in the cart, perhaps (6)
I believe the answer is:
tragic
'it's very serious' is the definition.
'joe's in the cart perhaps' is the wordplay.
'joe' becomes 'GI' (GI Joe).
'in the' indicates putting letters inside.
'perhaps' is an anagram indicator.
'cart' anagrammed gives 'trac'.
'gi' inserted inside 'trac' is 'TRAGIC'.
'when' is the link.
(Other definitions for tragic that I've seen before include "Intensely sad and distressing" , "Heart-rending" , "Pathetic" , "Disastrous, appalling" , "Characterised by extreme distress or sorrow" .)