Pip has Estella concealing periods (6)
I believe the answer is:
phases
'periods' is the definition.
(phase is a kind of period)
'pip has estella concealing' is the wordplay.
'concealing' indicates the answer is hidden within the clue.
'PHASES' is hidden within 'pip has estella'.
(Other definitions for phases that I've seen before include "Stages in process of change" , "Stages in processes" , "Shapes into stages in process" , "Distinct stages of development" .)