Initially fall in love with mass entertainment (4)
I believe the answer is:
film
'entertainment' is the definition.
(film is a type of entertainment)
'initially fall in love with mass' is the wordplay.
'initially' indicates taking the first letters.
'with' means one lot of letters go next to another.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
The initial letters of 'fall in love' is 'fil'.
'fil'+'m'='FILM'
(Other definitions for film that I've seen before include "Cinema celebrity" , "Skin" , "Thin layer - seen on screens" , "Screen" , "An entertainment; thin layer" .)