All wrong about some boys being faithful (5)
I believe the answer is:
loyal
'faithful' is the definition.
(similar in meaning)
'all wrong about some boys' is the wordplay.
'wrong' is an anagram indicator.
'about' is an insertion indicator.
'some' says to take the centre.
The centre of 'boys' is 'oy'.
'all' anagrammed gives 'lal'.
'lal' going around 'oy' is 'LOYAL'.
'being' is the link.
(Other definitions for loyal that I've seen before include "Staunch" , "Showing enduring firm support" , "Steadfast and true" , "Not perfidious" , "Faithful (in allegiance)" .)