Try underground in trade, reversing basic principles (6,5)

'basic principles' is the definition.
(a golden rule is a basic principle or law)

'try underground in trade reversing' is the wordplay.
'try' becomes 'go' (both can mean an attempt).
'underground' becomes 'urned' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
'trade' becomes 'sell' (to sell is to trade something for money).
'reversing' says the letters should be written backwards.
'urned' placed within 'sell' is 'selurnedl'.
'selurnedl' in reverse letter order is 'ldenrules'.
'go'+'ldenrules'='GOLDEN RULES'