Common prayer prefaces storm (7)
I believe the answer is:
average
'common' is the definition.
(I've seen this before)
'prayer prefaces storm' is the wordplay.
'prayer' becomes 'ave' (as in Ave Maria).
'prefaces' says to put letters next to each other (some letters go before others).
'storm' becomes 'rage' (synonyms).
'ave'+'rage'='AVERAGE'
(Other definitions for average that I've seen before include "Mean, so-so" , "building society customer loses capital over time" , "The statistical norm" , "It may be mean" , "par" .)