Father returns to Sparta, worried about union organisation (9)
I believe the answer is:
apparatus
'organisation' is the definition.
(I have seen 'political organisation' mean 'apparatus' so perhaps 'organisation' could also mean 'apparatus')
'father returns to sparta worried about union' is the wordplay.
'father returns' becomes 'ap' (I am not sure about this - if you are sure you should believe this answer much more).
'worried' indicates anagramming the letters (worry can mean to interfere with or disturb).
'about' means one lot of letters goes inside another.
'union' becomes 'U' (common abbreviation as in Trades Union Congress).
'sparta' with letters rearranged gives 'parats'.
'parats' going around 'u' is 'paratus'.
'ap'+'paratus'='APPARATUS'
Can you help me to learn more?
(Other definitions for apparatus that I've seen before include "Equipment, gear" , "Piece of equipment" , "Horse, maybe" , "Equipment designed for a specific function" , "Group of parts that work together to perform a function" .)