Girl or boy, one's a bit of alright (5)
I believe the answer is:
sonia
'girl' is the definition.
'boy one's a bit of alright' is the wordplay.
'boy' becomes 'son' (synonyms).
'one' becomes 'I' (Roman numeral).
'a bit of' indicates taking the first letters.
The initial letter of 'alright' is 'a'.
'son'+'i'+'a'='SONIA'
'or' is the link.
(Other definitions for sonia that I've seen before include "...O'Sullivan, great Irish athlete" , "Noa is really a girl" .)