Too American, having all systems go at last (4)
I believe the answer is:
also
'too' is the definition.
(synonyms)
'american having all systems go at last' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'having' means one lot of letters go next to another.
'at last' suggests the final letters.
The last letters of 'all systems go' are 'lso'.
'a'+'lso'='ALSO'
(Other definitions for also that I've seen before include "Additionally" , "an indication of more to come" , "in accompaniment" , "on top" , "To boot" .)