Branch of science tutors primarily presented in old films (6)
I believe the answer is:
optics
'branch of science' is the definition.
'tutors primarily presented in old films' is the wordplay.
'primarily' says to take the initial letters.
'presented in' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'films' becomes 'pics'.
The first letter of 'tutors' is 't'.
'o'+'pics'='opics'
't' placed into 'opics' is 'OPTICS'.
(Other definitions for optics that I've seen before include "Scientific studies of light and vision, the eyes" , "Science concerned with vision" , "Topics about eyes" , "Devices to measure out alcohol" , "Science of vision/light" .)