Portion of oil in this dressing (4)
I believe the answer is:
lint
'dressing' is the definition.
(I've seen this before)
'portion of oil in this' is the wordplay.
'portion of' indicates the answer is hidden within the clue.
'LINT' can be found hidden inside 'oil in this'.
(Other definitions for lint that I've seen before include "Wound-dressing fabric" , "Absorbent material for dressing wounds" , "Wound dressing" , "Soft cloth for dressing wounds" , "Fabric for dressing wounds" .)