Criminal lord keeping quiet for scheme (8)
I believe the answer is:
conspire
'for scheme' is the definition.
(I've seen this before)
'criminal lord keeping quiet' is the wordplay.
'criminal' becomes 'con' (abbreviation for 'convict').
'lord' becomes 'sire' (sire is a kind of lord).
'keeping' means one lot of letters goes inside another (some letters kept inside).
'quiet' becomes 'p' (music symbol for 'piano', ie soft or quiet).
'sire' going around 'p' is 'spire'.
'con'+'spire'='CONSPIRE'
(Other definitions for conspire that I've seen before include "Act secretly with others" , "Plot together - on prices (anag)" , "scheme" , "implicated in plot" , "Collude" .)