The hollow part of a flower that's opening provides proof (9)
I believe the answer is:
testament
'proof' is the definition.
(a testament to something is proof of it)
'the hollow part of a flower that's opening' is the wordplay.
'hollow' suggests removing the centre (the word is hollowed out).
'part of a flower' becomes 'stamen' ('stamen' is part of 'flower').
'opening' indicates taking the first letters.
'the' with its middle removed is 'te'.
The first letter of 'that' is 't'.
'te'+'stamen'+'t'='TESTAMENT'
'provides' is the link.
(Other definitions for testament that I've seen before include "with 1a [NEW] where is Mark?" , "Either of the two main divisions of the Bible" , "Biblical division" , "Statement (anag.)" , "document" .)