Two detectives, the second stopping sour outlaw (4,6)
I believe the answer is:
dick turpin
'outlaw' is the definition.
(I've seen this before)
'two detectives' is the wordplay.
'two detectives' can mean two replacements for 'detective'.
'detective' becomes 'dick' (informal term for a detective).
'detective' becomes 'turpin' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'dick'+'turpin'='DICK TURPIN'
'the second stopping sour' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for dick turpin that I've seen before include "Famous highwayman" , "outlaw" , "Eighteen century highwayman and robber" , "Notorious highwayman" , "Famous English crook, hanged at York in 1739" .)