Thomas to include a fruit (6)
I believe the answer is:
tomato
'fruit' is the definition.
(I've seen this before)
'thomas to include a' is the wordplay.
'thomas' becomes 'tom'.
'include' means one lot of letters goes inside another.
'tom'+'to'='tomto'
'tomto' enclosing 'a' is 'TOMATO'.
(Other definitions for tomato that I've seen before include "Salad fruit or berry" , "Also called the love apple" , ""Plum" or "beef" fruit?" , "that's suitable to eat" , "In France, it's a love apple" .)