Look sound when running easily (6)
I believe the answer is:
loping
'running easily' is the definition.
('lope' can be a synonym of 'run easily')
'look sound' is the wordplay.
'look' becomes 'lo' (archaic exclamation meaning 'look!').
'sound' becomes 'ping' (ping is a kind of sound).
'lo'+'ping'='LOPING'
'when' acts as a link.
(Other definitions for loping that I've seen before include "Walking with long strides" , "Running with long strides" .)