Opening success involving American and America (6)
I believe the answer is:
hiatus
'opening' is the definition.
(I've seen this before)
'success involving american and america' is the wordplay.
'success' becomes 'hit' (hit is a kind of success).
'involving' means one lot of letters goes inside another.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'and' means one lot of letters go next to another.
'america' becomes 'us' (abbreviation for the United States).
'hit' placed around 'a' is 'hiat'.
'hiat'+'us'='HIATUS'
(Other definitions for hiatus that I've seen before include "Break -- respite" , "chasm" , "Break, interruption" , "US hat I change for a break" , "Pause; discontinuity" .)