Well-to-do? That's laughable! (4)
I believe the answer is:
rich
I believe this is a double definition.
'well-to-do?' is the first definition.
('rich' can be similar in meaning to 'well-to-do')
'laughable' is the second definition.
(as in 'that's rich!', laughable or ridiculous)
'that's' is the link.
(Other definitions for rich that I've seen before include "Highly-flavoured" , "Full-flavoured" , "Wealthy - luxurious - fruitful" , "Affluent" , "Containing a lot of fat, eggs and sugar" .)