A horse, that is, for a little girl (5)

I believe the answer is:

aggie

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'little girl' is the definition.
'Aggie' can be an answer for 'girl'. I'm not sure about the 'little' bit.

'a horse that is for a' is the wordplay.
'a horse' becomes 'gg' (geegee or GG is a childish term for a horse).
'that is' becomes 'ie' (short for Latin 'id est', 'that is').
'for' means one lot of letters go next to another (I've seen this in other clues).
'gg'+'ie'='ggie'
'ggie' put after 'a' is 'AGGIE'.

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