Chemist placing vanadium in period (4)
I believe the answer is:
davy
'chemist' is the definition.
(Humphry Davy)
'vanadium in period' is the wordplay.
'vanadium' becomes 'v' (chemical symbol).
'in' indicates putting letters inside.
'period' becomes 'day' (day is a kind of period**).
'v' inserted within 'day' is 'DAVY'.
'placing' is the link.
This may not be right. It may belong to another bit of the clue.
(Other definitions for davy that I've seen before include "Miner's lamp inventor" , "Safety lamp inventor" .)