Method of counting back only partially useful at court (5)
I believe the answer is:
octal
'method of counting' is the definition.
(octal is a system of counting with 8 digits)
'back only partially useful at court' is the wordplay.
'back' is a reversal indicator.
'only partially' says the answer is hidden in the clue.
'latco' is hidden within 'useful at court'.
'latco' reversed gives 'OCTAL'.
(Other definitions for octal that I've seen before include "(Of maths) in base eight" , "type of base" , "(Of numbers) in base 8" , "with unusual base" , "using smaller base than usual" .)