Scientific study of scalp thus prodding nit (12)
I believe the answer is:
astrophysics
'scientific study' is the definition.
(branch of science)
'scalp thus prodding nit' is the wordplay.
'scalp' becomes 'trophy' (scalp can mean a battle trophy).
'thus' becomes 'sic' ('thus' in Latin).
'prodding' indicates putting letters inside (prod can mean to poke a hole into something).
'nit' becomes 'ass' (both can mean a foolish person).
'trophy'+'sic'='trophysic'
'trophysic' put into 'ass' is 'ASTROPHYSICS'.
'of' is the link.
(Other definitions for astrophysics that I've seen before include "Branch of science" , "Science of the properties of celestial bodies" , "Science of stars and galaxies" .)