Source of 21 data in much of volume 1 curiously safe outside (11)

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

Source of 21 data in much of volume 1, curiously safe outside *( VOLUM <e> + 1) in PETER (=safe); “much of” means that last letter only is dropped from anagram, indicated by “curiously”; a pluviometer is a rain gauge, hence “source of rain (=entry at 21) data”