Workers maybe covered in plant sap (4)
I believe the answer is:
ants
'workers maybe' is the definition.
(worker is a type of ant)
'covered in plant sap' is the wordplay.
'covered in' indicates a hidden word.
'ANTS' is hidden within 'plant sap'.
(Other definitions for ants that I've seen before include "Soldiers" , "Inhabitants of a formicary" , "They live in a formicary" , "24 [INSECT] s" , "Industrious insects" .)