The reason you finish in a legal action (5)
I believe the answer is:
cause
'the reason' is the definition.
(I've seen this in another clue)
'you finish in a legal action' is the wordplay.
'finish' suggests the final letters.
'in' means one lot of letters goes inside another.
'a legal action' becomes 'case' (a legal case).
The final letter of 'you' is 'u'.
'u' going into 'case' is 'CAUSE'.
(Other definitions for cause that I've seen before include "Charity - motive" , "Reason; belief" , "Give rise to" , "Make happen; something deserving support" , "A movement or charity- usually good" .)