Speak on British fruit tree (9)
I believe the answer is:
butternut
'tree' is the definition.
(butternut tree is a kind of tree)
'speak on british fruit' is the wordplay.
'speak' becomes 'utter' (synonyms).
'on' says to put letters next to each other.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'fruit' becomes 'nut' (nut is a kind of fruit).
'utter' put after 'b' is 'butter'.
'butter'+'nut'='BUTTERNUT'
(Other definitions for butternut that I've seen before include "sort of a squash" , "Wood" , "species of tree" , "Variety of pumpkin" .)