Odd man out: one must enter playing field (7)
I believe the answer is:
diamond
'playing field' is the definition.
(diamond is a kind of playing field)
'odd man out one must enter' is the wordplay.
'out' indicates anagramming the letters (out can mean wrong or inaccurate).
'one' becomes 'i' (Roman numeral).
'must enter' is an insertion indicator.
'odd'+'man'='oddman'
'oddman' is an anagram of 'damond'.
'damond' placed around 'i' is 'DIAMOND'.
(Other definitions for diamond that I've seen before include "Stone - card" , "Very hard gem" , "Carbon gemstone" , "Very hard substance" , "Birthstone for April" .)