Be second through the way out (5)
I believe the answer is:
exist
'be' is the definition.
(synonyms)
'second through the way out' is the wordplay.
'second' becomes 's' (common abbreviation).
'through' means one lot of letters goes inside another (inserted letters go through the word).
'the way out' becomes 'exit' ('exit' can be a synonym of 'way out').
's' put into 'exit' is 'EXIST'.
(Other definitions for exist that I've seen before include "Scrape by" , "Have being or continue living" , "Live" , "Have objective reality" , "Survive" .)