Dip (a biscuit) (4)
I believe the answer is:
dunk
'dip a biscuit' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dunk that I've seen before include "Dip (morsel) in liquid" , "Put into a liquid" , "Dip (eg, biscuit) in drink" , "Immerse (a biscuit)" , "Submerge in liquid" .)