Letters about alternative sources of decoration suggest the cuckoo pint (5,3,6)
I believe the answer is:
lords and ladies
'the cuckoo pint' is the definition.
(both can be names for the same plant)
'letters about alternative sources of decoration' is the wordplay.
'letters' becomes 'landladies' (people who let property).
'about' indicates putting letters inside.
'alternative' becomes 'or'.
'sources of decoration' becomes 'ds' (I can't explain this - if you can you should believe this answer much more).
'or'+'ds'='ords'
'landladies' placed around 'ords' is 'LORDS-AND-LADIES'.
'suggest' is the link.
I am not very happy about this link. It may be part of another bit of the clue.
(Other definitions for lords and ladies that I've seen before include "Wild flower" , "Bloomer" , "Peers" , "seen in another place?" , "plant" .)