Look in thick hair for fine powder (5)
I believe the answer is:
flour
'fine powder' is the definition.
(flour is a powdery substance)
'look in thick hair' is the wordplay.
'look' becomes 'lo' (archaic exclamation meaning 'look!').
'in' is an insertion indicator.
'thick hair' becomes 'fur' (I've seen this in another clue).
'lo' put into 'fur' is 'FLOUR'.
'for' is the link.
(Other definitions for flour that I've seen before include "Cake ingredient" , "Dusty white stuff" , "Finely ground grain" , "Main ingredient of bread" , "Fine-ground grain" .)