Conceal Jekyll's alter ego, you say? (4)
I believe the answer is:
hide
'conceal' is the definition.
('hide' can be a synonym of 'conceal')
'jekyll's alter ego you say?' is the wordplay.
'jekyll's alter ego' becomes 'hyde' (I've seen this before).
'you say?' shows a homophone (sound like).
'hyde' is a homophone of 'HIDE'.
(Other definitions for hide that I've seen before include "Take cover - skin" , "Place to observe wildlife" , "Once told, hurried < to > conceal" , "Pelt; conceal" , "Conceal - skin" .)