United behind after goal side advance (9)
I believe the answer is:
postulate
'advance' is the definition.
(thesaurus)
'united behind after goal side' is the wordplay.
'united' becomes 'u'.
'behind' is a charade indicator (letters next to each other).
'after' becomes 'post' (I've seen this before).
'goal side' becomes 'late' (I can't explain this - if you can you should give a lot more credence to this answer).
'u' after 'post' is 'postu'.
'postu'+'late'='POSTULATE'
Can you help me to learn more?
(Other definitions for postulate that I've seen before include "That which is assumed" , "Suppose" , "Put forward" , "Let sap out in order to claim something" , "Take for granted or lay down as self-evident" .)