Picture quietly stolen: old drawing by mathematician? (10)
I believe the answer is:
photograph
'picture' is the definition.
(photograph is a kind of picture)
'quietly stolen old drawing by mathematician?' is the wordplay.
'quietly' becomes 'p' (p for piano and quiet).
'stolen' becomes 'hot' (I've seen this before).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'drawing by mathematician?' becomes 'graph'.
'p'+'hot'+'o'+'graph'='PHOTOGRAPH'
(Other definitions for photograph that I've seen before include "Picture" , "Snapshot" .)