A sea-letter goes astray in landed property (4,6)
I believe the answer is:
real estate
'landed property' is the definition.
(I've seen this before)
'a sea-letter goes astray' is the wordplay.
'goes astray' is an anagram indicator.
'a'+'sea'+'letter'='asealetter'
'asealetter' with letters rearranged gives 'REAL ESTATE'.
'in' is the link.
(Other definitions for real estate that I've seen before include "Land (mainly US)" , "US-style spread?" , "Property, land (esp. US)" , "Land in America" , "land in New England" .)