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 a hidden word.
'INK' is hidden within '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" .)