Examination of American uniform (1,5)
I believe the answer is:
a level
'examination' is the definition.
(I've seen this before)
'american uniform' is the wordplay.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'uniform' becomes 'level' (I've seen this in another clue).
'a'+'level'='A-LEVEL'
'of' is the link.
(Other definitions for a level that I've seen before include "Advanced exam" , "School qualification" , "School examination in a single subject" , "Grade of exam" , "'British school examination (1,5)'" .)