Certain fluids can form a skin (4)
I believe the answer is:
inks
'certain fluids' is the definition.
'inks' can be an answer for 'fluids' (ink is a kind of fluid). I am not sure about the 'certain' bit.
'form a skin' is the wordplay.
'form' indicates an anagram (another form of the letters).
'skin' is an anagram of 'INKS'.
'can' acts as a link.
Can you help me to learn more?
(Other definitions for inks that I've seen before include "Fluids from cuttlefish" , "Printing fluids" , "Liquids used for printing" , "Printers' mediums" , "Sink beneath writing fluids" .)