Material noticed gathering outside of allotment (6)
I believe the answer is:
sateen
'material' is the definition.
(sateen is a kind of material)
'noticed gathering outside of allotment' is the wordplay.
'noticed' becomes 'seen' (I've seen this before).
'gathering' means one lot of letters goes inside another (gather can mean to acquire or bring in).
'outside of' says to hollow out the word (remove centre letters) (only letters on the outside of the word).
'allotment' with its centre removed is 'at'.
'seen' enclosing 'at' is 'SATEEN'.
(Other definitions for sateen that I've seen before include "Shiny cloth found in senate" , "Cotton fabric with a shiny finish" , "Sea net could provide a glossy fabric" , "Glossy cotton fabric" , "Cotton fabric with a satiny finish" .)