Crazy upset over young relative’s fruit-tree (6)
I believe the answer is:
damson
'fruit-tree' is the definition.
'crazy upset over young relative's' is the wordplay.
'crazy' becomes 'mad' (synonyms).
'upset over' says the letters should be written backwards (knocked over).
'young relative' becomes 'son' (a child).
'mad' written backwards gives 'dam'.
'dam'+'son'='DAMSON'
(Other definitions for damson that I've seen before include "might one be in a jam?" , "sort of fruit" , "Stone fruit for nomads" , "Wild plum" , "Acid purple plum" .)