Leader of Mexicans in America less (5)
I believe the answer is:
minus
'less' is the definition.
(I've seen this before)
'leader of mexicans in america' is the wordplay.
'leader of' indicates taking the first letters.
'america' becomes 'US' (United States).
The first letter of 'mexicans' is 'm'.
'm'+'in'+'us'='MINUS'
(Other definitions for minus that I've seen before include "Reduced by the subtraction of - thus?" , "Negative arithmetic operation" , "Disadvantage" , "deficiency" , "Less, without" .)