Religious people, heading off to see river (5)
I believe the answer is:
indus
'river' is the definition.
(Indian river)
'religious people heading off' is the wordplay.
'religious people' becomes 'hindus' (I've seen this before).
'heading off' means to remove the first letter.
'hindus' with its first letter removed is 'INDUS'.
'to see' is the link.
(Other definitions for indus that I've seen before include "banker" , "Main river of Pakistan" , "Pakistani river" , "running water" , "Important river" .)