Claim in a complicated legal letter (6)
I believe the answer is:
allege
'claim' is the definition.
(to allege something is to make a claim)
'in a complicated legal letter' is the wordplay.
'in' indicates putting letters inside.
'a complicated' indicates anagramming the letters (a complex or confusing ordering of the letters).
'letter' becomes 'e'.
'legal' anagrammed gives 'allge'.
'allge' enclosing 'e' is 'ALLEGE'.
(Other definitions for allege that I've seen before include "Declare or assert without proof" , "Assert, state without proof" , "Make a claim without proof" , "Claim, suggest" , "Declare without absolute proof" .)