Friends try to enter Siam in disguise (6)
I believe the answer is:
amigos
'friends' is the definition.
('friends' in Spanish)
'try to enter siam in disguise' is the wordplay.
'try' becomes 'go' (go is a kind of try).
'to enter' is an insertion indicator.
'in disguise' indicates an anagram.
'siam' anagrammed gives 'amis'.
'go' inserted inside 'amis' is 'AMIGOS'.
(Other definitions for amigos that I've seen before include "Friends (Spanish)" , "Friends in Spain" , "chums" , "Hispanic friends" , "Mexican pals" .)