Preserve horse for transporting mother and boy (9)
I believe the answer is:
marmalade
'preserve' is the definition.
(marmalade is a kind of preserve)
'horse for transporting mother and boy' is the wordplay.
'horse' becomes 'mare' (mare is a kind of horse**).
'for transporting' means one lot of letters goes inside another (a word carries or transports a set of letters).
'mother' becomes 'ma' (ma is a kind of mother).
'and' means one lot of letters go next to another.
'boy' becomes 'lad' (lad is a kind of boy).
'ma'+'lad'='malad'
'mare' placed around 'malad' is 'MARMALADE'.
(Other definitions for marmalade that I've seen before include "Orange conserve" , "confiture" , "Preserve made with citrus fruits" , "Sweet food" , "group?" .)