Great message about legal position, right (12)
I believe the answer is:
considerable
'great' is the definition.
(I've seen this before)
'message about legal position right' is the wordplay.
'message' becomes 'cable' (as in sending a cable to someone).
'about' means one lot of letters goes inside another.
'legal position' becomes 'onside' (a position allowed in football).
'right' becomes 'r' (common abbreviation).
'onside'+'r'='onsider'
'cable' placed around 'onsider' is 'CONSIDERABLE'.
(Other definitions for considerable that I've seen before include "of some importance" , "Fairly large or numerous" , "Tidy" , "A lot (of) - worthy of respect" , "worthy of attention" .)