Be dangerous, ultimately blocking the way out (5)
I believe the answer is:
exist
'be' is the definition.
(I know that be can be written as exist)
'dangerous ultimately blocking the way out' is the wordplay.
'ultimately' suggests the final letters.
'blocking' means one lot of letters goes inside another (to block can mean to plug a gap in something).
'the way out' becomes 'exit' ('exit' can be a synonym of 'way out').
The final letter of 'dangerous' is 's'.
's' inserted inside 'exit' is 'EXIST'.
(Other definitions for exist that I've seen before include "Get a life!" , "Live" , "Be alive" , "quick!" , "Have being or continue living" .)