Musical theme in mass sacred work provided (5)
I believe the answer is:
motif
'musical theme' is the definition.
(motif is a kind of musical theme)
'mass sacred work provided' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'sacred work' becomes 'OT' (Old Testament).
'provided' becomes 'if' (eg 'provided it's sunny' means 'if it's sunny').
'm'+'ot'+'if'='MOTIF'
'in' is the link.
(Other definitions for motif that I've seen before include "Theme or melody" , "Main theme in music" , "sign" , "Recurrent theme in music..." , "Theme in music or book" .)