Reason watch is kept on private clubs (5)
I believe the answer is:
logic
'reason' is the definition.
(logic is using reason)
'watch is kept on private clubs' is the wordplay.
'watch' becomes 'lo' (archaic exclamation meaning 'look!').
'is kept on private' becomes 'gi' (I can't justify this - if you can you should give a lot more credence to this answer).
'clubs' becomes 'c' (abbreviation in card games).
'lo'+'gi'+'c'='LOGIC'
Can you help me to learn more?
(Other definitions for logic that I've seen before include "Good reason" , "using premises" , "The science of reasoning from cause to effect" , "Reasonable judgement" , "Reasoned argument" .)