Sour thing, finding crustacean on tree (4,5)
I believe the answer is:
crab apple
'sour thing' is the definition.
'crab apple' can be an answer for 'sour' (thesaurus). I'm not certain of the 'thing' bit.
'crustacean on tree' is the wordplay.
'crustacean' becomes 'crab' (crab is a kind of crustacean).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'tree' becomes 'apple' (apple is a kind of tree).
'crab'+'apple'='CRAB-APPLE'
'finding' is the link.
Can you help me to learn more?
(Other definitions for crab apple that I've seen before include "unappetizing fruit" , "A small sour fruit - will it keep the doctor away?" , "A small sour fruit, part crustacean" , "Fruit tree with sour bright red crop" , "Small sour version of common orchard fruit" .)