Relative welcomes finish by forward (4,2)
I believe the answer is:
Here is my best explanation:
'forward' is the definition.
('send on' can be a synonym of 'forward')
'relative welcomes finish' is the subsidiary indication.
'relative' becomes 'son' (son is a kind of relative**).
'welcomes' is an insertion indicator (in sense of 'invites in').
'finish' becomes 'end' ('end' can be a synonym of 'finish').
'son' enclosing 'end' is 'sendon'.
'by' is the link.