A firm originally retailing nourishing fruit (5)
I believe the answer is:
acorn
'fruit' is the definition.
(acorn is a kind of fruit)
'a firm originally retailing nourishing' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'originally' suggests taking the first letters.
The first letters of 'retailing nourishing' is 'rn'.
'a'+'co'+'rn'='ACORN'
(Other definitions for acorn that I've seen before include "Potentially future tree" , "cup's content" , "mast" , "Tree seed" , "old computer?" .)