A little silent about old retired detectives (7)
I believe the answer is:
modicum
'a little' is the definition.
(I've seen this before)
'silent about old retired detectives' is the wordplay.
'silent' becomes 'mum' (keep mum).
'about' is an insertion indicator.
'old' becomes 'o' (abbreviation).
'retired' says the letters should be written in reverse.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'cid' backwards is 'dic'.
'o'+'dic'='odic'
'mum' placed around 'odic' is 'MODICUM'.
(Other definitions for modicum that I've seen before include "dash" , "Tad" , "I cod Mum into providing a small quantity" , "Moderate quantity" , "A small portion" .)