We fuss about length and line? That's rich! (4-2-2)
I believe the answer is:
Here is my best explanation:
'rich' is the definition.
('well-to-do' can be similar in meaning to 'rich')
'we fuss about length and line?' is the wordplay.
'fuss' becomes 'todo' (I've seen this before).
'about' says to put letters next to each other.
'length' becomes 'l'.
'and' says to put letters next to each other (I've seen this in other clues).
'line?' becomes 'l'.
'todo' put after 'l' is 'ltodo'.
'ltodo' after 'l' is 'lltodo'.
'that's' is the link.