Big bottle with a banner on (6)
I believe the answer is:
flagon
'big bottle' is the definition.
'flagon' can be an answer for 'bottle' (I've seen this before). I'm not certain of the 'big' bit.
'with a banner on' is the wordplay.
'with' says to put letters next to each other.
'a banner' becomes 'flag' (I've seen this in another clue).
'flag'+'on' is 'FLAGON'.
Can you help me to learn more?
(Other definitions for flagon that I've seen before include "Large container for serving drink" , "Wine jug" , "Large round bottle as for cider" , "Large wine-bottle, jug" , "Drinking vessel" .)