Top figure in the main almost entirely behind attitude about historic currency (7)
I believe the answer is:
admiral
'top figure in the main' is the definition.
(the main can mean the sea)
'almost entirely behind attitude about historic currency' is the wordplay.
'almost entirely' becomes 'al' (I can't justify this - if you can you should believe this answer much more).
'behind' means one lot of letters go next to another.
'attitude' becomes 'air' (I've seen this before).
'about' is an insertion indicator.
'historic currency' becomes 'DM' (Deutsche Mark, former German currency).
'air' going around 'dm' is 'admir'.
'al' put after 'admir' is 'ADMIRAL'.
(Other definitions for admiral that I've seen before include "[NELSON] perhaps" , "Naval officer of high rank" , "Navy rank" , "Benbow, say" , "Brightly coloured butterfly" .)