Agent caught in attack returning to get plaster? (6)
I believe the answer is:
gypsum
'plaster?' is the definition.
(mineral used to make plaster)
'agent caught in attack returning' is the wordplay.
'agent' becomes 'spy' (spy is a kind of agent).
'caught in' means one lot of letters goes inside another.
'attack' becomes 'mug' (to mug someone is to attack and rob them).
'returning' says the letters should be written in reverse.
'spy' inserted inside 'mug' is 'muspyg'.
'muspyg' reversed gives 'GYPSUM'.
'to get' acts as a link.
(Other definitions for gypsum that I've seen before include "Soft white or grey mineral" , "Ingredient of plaster of Paris" , "Ingredient of fertilizer" , "Base of plaster of Paris" , "Plaster of Paris is made from this chalk-like mineral" .)