American flower in Rhode Island, giant swallows also (3,6)
I believe the answer is:
rio grande
'american flower' is the definition.
(I've seen this before)
'rhode island giant swallows also' is the wordplay.
'rhode island' becomes 'ri' (US postal abbreviation).
'giant' becomes 'ogre' (ogre is a kind of giant).
'swallows' is an insertion indicator.
'also' becomes 'and' (I've seen this before).
'ogre' placed around 'and' is 'ogrande'.
'ri'+'ogrande'='RIO GRANDE'
'in' acts as a link.
(Other definitions for rio grande that I've seen before include "American flower" , "US-Mexico border river" , "River flowing between USA and Mexico" , "Long river" , "running water" .)