Cry buckets -- thick tissue! (7)
I believe the answer is:
blubber
'tissue' is the definition.
(I've seen this before)
'cry buckets thick' is the wordplay.
'cry' becomes 'blub' (I've seen this before).
'buckets thick' becomes 'ber' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'blub'+'ber'='BLUBBER'
Can you help me to learn more?
(Other definitions for blubber that I've seen before include "Fatty tissue of a whale" , "Sob loudly" , "Fat of sea mammals - cry" , "source of oil" , "Layer of fat under the skin of whales" .)