Relative welcomes finish by forward (4,2)

'forward' is the definition.
('send on' can be a synonym of 'forward')

'relative welcomes finish' is the wordplay.
'relative' becomes 'son' (son is a kind of relative).
'welcomes' indicates putting letters inside (in sense of 'invites in').
'finish' becomes 'end' ('end' can be a synonym of 'finish').
'son' placed around 'end' is 'SEND ON'.

'by' is the link.