When upset, flake out (6)
I believe the answer is:
asleep
'out' is the definition.
(out can mean unconscious or asleep)
'when upset flake' is the wordplay.
'when' becomes 'as' (eg 'when I walked' means 'as I walked').
'upset' is a reversal indicator.
'flake' becomes 'peel' (synonyms).
'peel' backwards is 'leep'.
'as'+'leep'='ASLEEP'
(Other definitions for asleep that I've seen before include "(Of a limb) numb" , "Dead to the world" , "Please (anag) - numb" , "Elapse (anag)" , "Having nodded off" .)