Metalworker is in time to work with this medium (8)
I believe the answer is:
tinsmith
'metalworker' is the definition.
(I know that tinsmith is a type of blacksmith)
'in time to work with this medium' is the wordplay.
'time' becomes 't' (abbreviation).
'to work with' indicates anagramming the letters (letters worked into a new order).
'medium' becomes 'm' (as in UK clothing sizes).
'in'+'t'='int'
'this'+'m'='thism'
'int' with its letters mixed up with 'thism' makes 'TINSMITH'.
'is' is the link.
(Other definitions for tinsmith that I've seen before include "Sn worker" , "A craftsman" , "Possible forger" , "metalworker?" , "Maker of articles with metal, symbol Sn" .)