Fluid used in various applications (3)
I believe the answer is:
sap
'fluid' is the definition.
(I have seen 'Vital fluid ' mean 'sap' so perhaps 'fluid' could also mean 'sap')
'in various applications' is the wordplay.
'in' indicates a hidden word.
'SAP' can be found hidden inside 'various applications'.
'used' acts as a link.
(Other definitions for sap that I've seen before include "Undermine the tree juice" , "Plant liquid or general vitality" , "Watery substance in a plant" , "Plant juice; undermine" , "The juice in trees" .)