One with spoilt mentality personally (10)
I believe the answer is:
intimately
'personally' is the definition.
(as in to know someone personally or intimately)
'one with spoilt mentality' is the wordplay.
'one' becomes 'I' (Roman numeral).
'with' is a charade indicator (letters next to each other).
'spoilt' is an anagram indicator.
'mentality' is an anagram of 'ntimately'.
'i'+'ntimately'='INTIMATELY'
(Other definitions for intimately that I've seen before include "in a familiar way" , "Privately" .)