Slogan arises after opening of British base (6)
I believe the answer is:
bottom
'base' is the definition.
(I've seen this before)
'slogan arises after opening of british' is the wordplay.
'slogan' becomes 'motto' (synonyms).
'arises' is a reversal indicator (I've seen 'arising' mean this).
'after' says to put letters next to each other.
'opening of british' becomes 'b' (I can't explain this - if you can you should give a lot more credence to this answer).
'motto' in reverse letter order is 'ottom'.
'ottom' put after 'b' is 'BOTTOM'.
Can you help me to learn more?
(Other definitions for bottom that I've seen before include "Lowest point - weaver" , "Base; Shakespearean character" , "Mechanical" , "Athenian weaver (AMND)" , "Shakespearean weaver" .)