Decay rampant and in American buildings (8)
I believe the answer is:
rotundas
'buildings' is the definition.
(rotunda is a kind of building)
'decay rampant and in american' is the wordplay.
'decay' becomes 'rot' (rotting is a kind of decaying).
'rampant' indicates anagramming the letters (I've seen this in other clues).
'in' is an insertion indicator.
'american' becomes 'us'.
'and' anagrammed gives 'nda'.
'nda' inserted within 'us' is 'undas'.
'rot'+'undas'='ROTUNDAS'
(Other definitions for rotundas that I've seen before include "Domed buildings" , "Circular buildings with domes" , "Round buildings, usually with domes" .)