Most fastidious theologian that is participating in Lent? (8)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

An envelope (‘participating in’) of DD ( Divinitatis Doctor, Doctor of Divinity, ‘theologian’) plus I.E. (‘that is’) in FAST (‘Lent’). The definition strikes me as questionable, although faddy as picky about food is somewhere close.