Engrossing period obligatory (12)
I believe the answer is:
spellbinding
'engrossing' is the definition.
(I've seen this before)
'period obligatory' is the wordplay.
'period' becomes 'spell' (spell is a kind of period).
'obligatory' becomes 'binding' (I've seen this before**).
'spell'+'binding'='SPELLBINDING'
(Other definitions for spellbinding that I've seen before include "Engrossing" , "Fascinating" , "Enthralling" .)