Criminal grabbing too much material (6)
I believe the answer is:
cotton
'material' is the definition.
(cotton is a kind of material)
'criminal grabbing too much' is the wordplay.
'criminal' becomes 'con' (abbreviation for 'convict').
'grabbing' indicates putting letters inside.
'too much' becomes 'OTT' (short for over the top).
'con' placed around 'ott' is 'COTTON'.
(Other definitions for cotton that I've seen before include "Plant fibre for fabrics" , "Plant, its fibres for yarn" , "Plantation product" , "This wool is a fluffy wadding" , "Substance like fine wool" .)