Source of oval fruit, case of timber in that? (3-4)
I believe the answer is:
oak-tree
'source' is the definition.
(I have seen 'Source of acorns' mean 'oak tree' so perhaps 'source' could also mean 'oak tree')
'oval fruit case of timber in that?' is the wordplay.
'oval' becomes 'O'.
'fruit' becomes 'akee' (akee is a kind of fruit).
'case of' means to remove the middle letters (outsides of).
'in that?' is an insertion indicator.
'timber' with its centre removed is 'tr'.
'akee' placed around 'tr' is 'aktree'.
'o'+'aktree'='OAK-TREE'
'of' acts as a link.
(Other definitions for oak-tree that I've seen before include "Matured acorn" , "It grows mighty from an acorn" , "Old plant (round which to tie a yellow ribbon?)" , "Product of acorn" , "Acorn plant" .)