Honestly, it's not down back! (7)
I believe the answer is:
upfront
'honestly' is the definition.
I don't know anything about this answer so I can't judge whether this works.
'not down back' is the wordplay.
'not down' becomes 'up' ('up' is the opposite to 'down').
'back' becomes 'front' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'up'+'front'='UPFRONT'
'it's' is the link.
Can you help me to learn more?
(Other definitions for upfront that I've seen before include "(Of payment) in advance" , "Direct" , "Bold, frank (colloq.)" , "Informally, straightforward or frank" , "Straightforward or prominent" .)