In court, claim legal trickery's a foregone conclusion! (6)
I believe the answer is:
allege
'in court claim' is the definition.
'allege' can be an answer for 'claim' (to allege something is to make a claim). I am not certain of the 'in court' bit.
'legal trickery's a foregone conclusion' is the wordplay.
'trickery's' indicates anagramming the letters (I've seen 'trickery' mean this).
'conclusion' indicates one should take the final letters.
The last letter of 'foregone' is 'e'.
'legal' with letters rearranged gives 'alleg'.
'alleg'+'e'='ALLEGE'
Can you help me to learn more?
(Other definitions for allege that I've seen before include "Swear" , "State as a fact without proof" , "Claim (a person is guilty)" , "hold" , "Declare without absolute proof" .)