They make light of upsets (6)
I believe the answer is:
spills
'they' is the definition.
Both the answer and definition are plural nouns.
Maybe they are linked in a way I don't understand?
'light of upsets' is the wordplay.
'light of' becomes 'sp' (I am not sure about this - if you are sure you should believe this answer much more).
'upsets' becomes 'ills' (ill is a kind of upset).
'sp'+'ills'='SPILLS'
'make' is the link.
Can you help me to learn more?
(Other definitions for spills that I've seen before include "Pieces of rolled paper used to light fires" , "Pieces of rolled paper used to light a fire" , "Causes liquid to flow out" .)