Gunners upset by a lot of Celtic's rivals — fix! (7)
I believe the answer is:
arrange
'fix' is the definition.
(I've seen this before)
'gunners upset by a lot of celtic's rivals' is the wordplay.
'gunners' becomes 'RA' (Royal Artillery).
'upset' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'a lot of celtic's rivals' becomes 'range' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'ra' back-to-front is 'ar'.
'ar'+'range'='ARRANGE'
Can you help me to learn more?
(Other definitions for arrange that I've seen before include "Set in order" , "Orchestrate - order" , "Order, plan or design" , "Rare nag (anag.)" , "Settle, work out" .)