Base, initially perhaps long, is not that heavy (6)
I believe the answer is:
plinth
'base' is the definition.
(I've seen this before)
'initially perhaps long is not that heavy' is the wordplay.
'initially' says to take the initial letters.
The initial letters of 'perhaps long is not that heavy' is 'PLINTH'.
(Other definitions for plinth that I've seen before include "Base for, eg, statue" , "Pedestal for a statue" , "Base supporting a statue" , "Stand" , "Architectural support or base" .)