Criminals concerned with concealing very good plot (8)
I believe the answer is:
conspire
'plot' is the definition.
(conspiring is a kind of plotting)
'criminals concerned with concealing very good' is the wordplay.
'criminals' becomes 'cons'.
'concerned with' becomes 're'.
'concealing' indicates putting letters inside.
'very good' becomes 'pi' (I've seen this in other clues**).
'cons'+'re'='consre'
'consre' going around 'pi' is 'CONSPIRE'.
(Other definitions for conspire that I've seen before include "Act secretly with others" , "'Plot, connive (8)'" , "So prince is given to plan secretly with others" , "Plot together - on prices (anag)" , "Secretly plan together" .)