Crack to extend, doing for flower (10)
I believe the answer is:
snapdragon
'flower' is the definition.
(snapdragon is a kind of flower)
'crack to extend doing' is the wordplay.
'crack' becomes 'snap' ('snap' can be a synonym of 'crack').
'to extend doing' becomes 'dragon' (I can't explain this - if you can you should give a lot more credence to this answer).
'snap'+'dragon'='SNAPDRAGON'
'for' is the link.
Can you help me to learn more?
(Other definitions for snapdragon that I've seen before include "that's in the garden?" , "one found in bed" , "Kind of antirrhinum" , "flower" , "one's found in the garden" .)