An arm casually wrapped round American love, sweet thing (8)
I believe the answer is:
marzipan
'sweet thing' is the definition.
(marzipan is sweet)
'an arm casually wrapped round american love' is the wordplay.
'casually' indicates an anagram (a casual or disordered version of the letters).
'wrapped round' indicates putting letters inside.
'american love' becomes 'zip' (US term for zero or nothing).
'an'+'arm'='anarm'
'anarm' with letters rearranged gives 'maran'.
'maran' placed around 'zip' is 'MARZIPAN'.
(Other definitions for marzipan that I've seen before include "Marchpane" , "Almond-based cake covering" , "sort of sweet" , "Kind of icing, nice on cakes" , "Paste on cake" .)