Fluid of a certain kind (3)
I believe the answer is:
ink
'fluid' is the definition.
(ink is a kind of fluid)
'of a certain kind' is the wordplay.
'of' indicates the answer is hidden within the clue.
'INK' can be found hidden inside 'certain kind'.
(Other definitions for ink that I've seen before include "Cuttlefish's secretion" , "Liquid used in printing" , "Printer's medium" , "Material used in printing" , "Squid's means of concealment" .)