Rush as then, possibly (6)
I believe the answer is:
hasten
'rush' is the definition.
(I know that rush can be written as hasten)
'as then possibly' is the wordplay.
'possibly' indicates an anagram.
'as'+'then'='asthen'
'asthen' anagrammed gives 'HASTEN'.
(Other definitions for hasten that I've seen before include "Be quick to do something" , "Step up" , "dash" , "Make faster" , "Hurry along" .)