Unusually par the first: golf club detailed round without experience (1,6)
I believe the answer is:
a priori
'without experience' is the definition.
'unusually par the first golf club detailed round' is the wordplay.
'unusually' indicates anagramming the letters.
'the first' becomes 'i' (I in Roman numerals eg Elizabeth I).
'golf club' becomes 'iron' (I've seen this before).
'detailed' means to remove the last letter (the word must be end-less or 'de-tailed').
'round' is a reversal indicator.
'iron' with its final letter removed is 'iro'.
'iro' back-to-front is 'ori'.
'par' with letters rearranged gives 'apr'.
'apr'+'i'+'ori'='A PRIORI'
(Other definitions for a priori that I've seen before include "(Of reasoning) deduction from a general principle" , "(Reasoning) from first principles (Latin)" , "arguing from first principles" , "Based on theory (Latin)" , "Way of reasoning" .)