Work hard in America after start of exam, worried it's complex (9)
I believe the answer is:
elaborate
'it's complex' is the definition.
(I've seen this before)
'work hard in america after start of exam worried' is the wordplay.
'work hard in america' becomes 'labor' (US spelling of 'labour').
'after' means one lot of letters go next to another.
'start of' suggests taking the first letters.
'worried' becomes 'ate' (something worrying may eat someone up).
The initial letter of 'exam' is 'e'.
'labor' put after 'e' is 'elabor'.
'elabor'+'ate'='ELABORATE'
(Other definitions for elaborate that I've seen before include "Expand (upon)" , "Complicated or ornate" , "with added detail" , "Work out or describe in detail" , "With many parts or details, maybe a pattern" .)