Mother hosting police ball is upset? Just a bit (7)
I believe the answer is:
modicum
'a bit' is the definition.
(I've seen this before)
'mother hosting police ball is upset?' is the wordplay.
'mother' becomes 'mum' (informal term for mother).
'hosting' means one lot of letters goes inside another (inserted letters are welcomed in or hosted).
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'ball' becomes 'o' (O looks like a ball).
'is upset?' says the letters should be written backwards (I've seen 'being upset' mean this).
'cid'+'o'='cido'
'mum' enclosing 'cido' is 'mucidom'.
'mucidom' in reverse letter order is 'MODICUM'.
'just' acts as a link.
(Other definitions for modicum that I've seen before include "Moderate quantity" , "I cod Mum into providing a small quantity" , "suggestion" , "A small amount as of drink" , "Tad" .)