Function to make leather (3)
I believe the answer is:
tan
I believe this is a double definition.
'function' is the first definition.
(tangent, mathematical function)
'make leather' is the second definition.
'to' is the link.
(Other definitions for tan that I've seen before include "Process" , "Yellowish-brown colour" , "Turn dark in sun" , "Brown to beat" , "Thrash" .)