Criminal grabbing too much material (6)
I believe the answer is:
cotton
'material' is the definition.
(I know that cotton is a type of material)
'criminal grabbing too much' is the wordplay.
'criminal' becomes 'con' (abbreviation for 'convict').
'grabbing' is an insertion indicator.
'too much' becomes 'OTT' (short for over the top).
'con' going 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" .)