Understand saying 'swing both ways' (6)
I believe the answer is:
seesaw
'swing both ways' is the definition.
'understand saying' is the wordplay.
'understand' becomes 'see' ('see' can be a synonym of 'understand').
'saying' becomes 'saw' (saw is a kind of saying).
'see'+'saw'='SEESAW'
(Other definitions for seesaw that I've seen before include "Recreation ground item" , "Rise and fall" , "Fluctuating playground ride" , "Playground furniture" , "Children's plaything with plank over barrel" .)