It's grossly overpriced - by a tearaway? (3-3)
I believe the answer is:
rip-off
'it's grossly' is the definition.
I can't tell whether this defines the answer.
'overpriced by a tearaway?' is the wordplay.
'overpriced' becomes 'off' (I can't explain this - if you can you should give a lot more credence to this answer).
'by' is a charade indicator (letters next to each other).
'a tearaway?' becomes 'rip' (I've seen this before).
'off' put after 'rip' is 'RIP-OFF'.
Can you help me to learn more?
(Other definitions for rip-off that I've seen before include "Scam" , "Act of fraud or swindle (inf)" , "Greatly overpriced item" , "Grossly overpriced item" , "Con trick" .)