A small amount of old detectives rose in silence (7)
I believe the answer is:
modicum
'a small amount' is the definition.
(I've seen this before)
'old detectives rose in silence' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'detectives' becomes 'cid'.
'rose' says the letters should be written in reverse.
'in' is an insertion indicator.
'silence' becomes 'mum' (mum is a kind of silence).
'cid' back-to-front is 'dic'.
'o'+'dic'='odic'
'odic' placed into 'mum' is 'MODICUM'.
'of' acts as a link.
(Other definitions for modicum that I've seen before include "Little bit" , "touch" , "I cod Mum into providing a small quantity" , "Tad" , "dash" .)