Guide leaving town — he knows what's coming (4)
I believe the answer is:
seer
'he knows what's coming' is the definition.
(someone who can see the future)
'guide leaving town' is the wordplay.
'guide' becomes 'steer' (synonyms).
'leaving' is a deletion indicator.
'town' becomes 't' (abbreviation for town).
'steer' with 't' taken out is 'SEER'.
(Other definitions for seer that I've seen before include "One having visions of the future" , "Prophet, one who has visions" , "Prophet or visionary" , "Fortune-teller" , "One who knows the future" .)