Quick! It's liable to break! (6)
I believe the answer is:
snappy
'quick' is the definition.
(snappy means very fast)
'liable to break' is the wordplay.
'liable' becomes 'py' (I can't justify this - if you can you should believe this answer much more).
'to' is a charade indicator (letters next to each other) (I've seen this in other clues).
'break' becomes 'snap' (snapping is a kind of breaking).
'py' put after 'snap' is 'SNAPPY'.
'it's' acts as a link.
Can you help me to learn more?
(Other definitions for snappy that I've seen before include "Cleverly concise; stylish" , "Irritable, curt; concise" , "Quick and energetic" , "Irritable, curt; stylish" , "Smart - lively" .)