Head of research and a professor in their element (5)
I believe the answer is:
radon
'element' is the definition.
(chemical element)
'head of research and a professor' is the wordplay.
'head of' suggests taking the first letters.
'a professor' becomes 'don' (term for a university professor).
The first letters of 'research and' is 'ra'.
'ra'+'don'='RADON'
'in their' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for radon that I've seen before include "gets no reaction" , "Colourless gas, atomic number 86" , "Radioactive gas, linked to granite" , "Gaseous element, Rn" , "Element, symbol Rn" .)