Train from South Herts town (6)
I believe the answer is:
string
'train' is the definition.
('string' can be a synonym of 'train')
'south herts town' is the wordplay.
'south' becomes 's' ('S' can be a synonym of 'south').
'herts town' becomes 'tring' (I've seen this before).
's'+'tring'='STRING'
'from' is the link.
(Other definitions for string that I've seen before include "Collectively, the racehorses trained at one stable" , "Procession" , "might one be plucked?" , "Number of racehorses" , "Succession" .)