Understand start of gruesome film (3)
I believe the answer is:
get
'understand' is the definition.
(getting is a kind of understanding)
'start of gruesome film' is the wordplay.
'start of' suggests taking the first letters.
'film' becomes 'et' (**).
The initial letter of 'gruesome' is 'g'.
'g'+'et'='GET'
(Other definitions for get that I've seen before include "Escape" , "Understand" , "Fetch or obtain" , "See 18" , "Procure or secure" .)