Actually it's about a friend (6)
I believe the answer is:
really
'actually' is the definition.
(I know that actually can be written as really)
'about a friend' is the wordplay.
'about' becomes 're' ('re' means regarding or about).
'a friend' becomes 'ally' (an ally is a friend or supporter).
're'+'ally'='REALLY'
'it's' is the link.
(Other definitions for really that I've seen before include "surprised expression" , "Indeed" , "without a doubt" , "Actually, truly" , "Honestly, it's a fact" .)