Understand playground game upset that girl (6)
I believe the answer is:
gather
'understand' is the definition.
('gather' can be a synonym of 'understand')
'playground game upset that girl' is the wordplay.
'playground game' becomes 'tag'.
'upset' says the letters should be written backwards.
'that girl' becomes 'her'.
'tag' backwards is 'gat'.
'gat'+'her'='GATHER'
(Other definitions for gather that I've seen before include "Harvest; understand" , "From information given, learn" , "Collect together in one place" , "Assemble - conclude - contract" , "Harvest or learn by inference" .)