Cautious little girl turning up to support little boy (5)
I believe the answer is:
timid
'cautious' is the definition.
(synonyms)
'little girl turning up to support little boy' is the wordplay.
'little girl' becomes 'di' (short for Diane, Diana etc).
'turning up' says the letters should be written in reverse.
'to support' is a charade indicator (letters next to each other) (in a down clue, some letters hold others up).
'little boy' becomes 'tim'.
'di' written backwards gives 'id'.
'id' put after 'tim' is 'TIMID'.
(Other definitions for timid that I've seen before include "afraid to speak out?" , "Not assertive, retiring" , "Not brave" , "Inclined to avoid people" , "Pusillanimous" .)