Playground quarrel — four sides in it? (9)
I believe the answer is:
rectangle
'four sides in it?' is the definition.
(four sided shape)
'playground quarrel' is the wordplay.
'playground' becomes 'rec' (short for recreation ground).
'quarrel' becomes 'tangle' (tangle can mean a quarrel or argument).
'rec'+'tangle'='RECTANGLE'
(Other definitions for rectangle that I've seen before include "box" , "Parallelogram with four right angles" , "Geometric figure with four right angles, not square" , "Quadrilateral" , "Four-sided figure" .)