Trees once again kept still initially (4)
I believe the answer is:
oaks
'trees' is the definition.
(oak is a kind of tree)
'once again kept still initially' is the wordplay.
'initially' indicates taking the first letters.
The initial letters of 'once again kept still' is 'OAKS'.
(Other definitions for oaks that I've seen before include "Hardwood trees" , "Horse race - trees" , "Acorn producers" , "material for shipbuilding" , "Trees of the genus Quercus" .)