One page in dictionary (American) is kind of complex (7)
I believe the answer is:
oedipus
'is kind of complex' is the definition.
(I've seen this in another clue)
'one page in dictionary american' is the wordplay.
'one' becomes 'i' (Roman numeral).
'page' becomes 'p'.
'in' is an insertion indicator.
'dictionary' becomes 'OED' (Oxford English Dictionary).
'american' becomes 'US' (United States).
'i'+'p'='ip'
'oed'+'us'='oedus'
'ip' placed inside 'oedus' is 'OEDIPUS'.
(Other definitions for oedipus that I've seen before include "Theban king of legend" , "Theban who married his mother" , "Mythological king who killed his father and married his mother" , "Mythical Greek who killed his father and married his mother" , "Tragic king of Thebes (with a complex)" .)