Support son, overweight, knocked back by female (5)
I believe the answer is:
Here is my best explanation:
'support' is the definition.
'son overweight knocked back by female' is the subsidiary indication.
'son' becomes 's'.
'overweight' becomes 'fat' ('fat' can be similar in meaning to 'overweight').
'knocked back' shows that the letters should be reversed in order.
'by' means one lot of letters go next to another.
'female' becomes 'f' (abbreviation).
'fat' written backwards gives 'taf'.
(Other definitions for staff that I've seen before include "Rod of authority", "Rod - workforce", "man", "Hands", "support for notes", "team?", "Employees; sceptre", "Workforce", "Employees - stick" and "Stick of authority".)