Crossword Genius

Engrossing period obligatory (12)

Ross

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" .)

I've seen this clue in The Telegraph.
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