Whisper when saint breaks law (6)
I believe the answer is:
rustle
'whisper' is the definition.
('rustle' can be a synonym of 'whisper')
'saint breaks law' is the wordplay.
'saint' becomes 'st'.
'breaks' means one lot of letters goes inside another.
'law' becomes 'rule' (rule is a kind of law).
'st' inserted inside 'rule' is 'RUSTLE'.
'when' acts as a link.
(Other definitions for rustle that I've seen before include "Steal livestock - make a little noise" , "making little noise" , "Steal (cattle, etc)" , "Swish - lustre (anag)" , "Make sound as of paper crumpling - or steal cattle" .)