Trinity Hall, reading electrical engineering, student starts delving into book (9)
I believe the answer is:
threesome
'trinity' is the definition.
('threesome' can be a synonym of 'trinity')
'hall reading electrical engineering student starts delving into book' is the wordplay.
'starts' indicates taking the first letters.
'delving into' means one lot of letters goes inside another.
'book' becomes 'tome' (tome is a kind of book**).
The initial letters of 'hall reading electrical engineering student' is 'hrees'.
'hrees' placed into 'tome' is 'THREESOME'.
(Other definitions for threesome that I've seen before include "Horse meet (anag.)" , "Chekhov's sisters" , "Freeman Hardy Willis, say" , "Magi" , "The Fates, perhaps" .)