Burn saint concerning a mass (6)
I believe the answer is:
stream
'burn' is the definition.
(burn is a Scottish dialect word for a stream)
'saint concerning a mass' is the wordplay.
'saint' becomes 'st'.
'concerning' becomes 're'.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'st'+'re'+'a'+'m'='STREAM'
(Other definitions for stream that I've seen before include "Brook, burn" , "flower" , "selected pupils" , "Small narrow river" , "divide pupils into ability groups" .)