Nowadays, space station has a top lady fleet leader (7)
I believe the answer is:
admiral
'fleet leader' is the definition.
(I've seen this in another clue)
'nowadays space station has a top lady' is the wordplay.
'nowadays' becomes 'AD' (AD, Anno Domini, as opposed to BC, Before Christ).
'space station' becomes 'Mir' (I've seen this in another clue).
'has' is a charade indicator (letters next to each other).
'top' says to take the initial letters (in a down clue, the first letter is top).
The initial letter of 'lady' is 'l'.
'ad'+'mir'+'a'+'l'='ADMIRAL'
(Other definitions for admiral that I've seen before include "Naval high-up" , "Navy rank" , "Brightly coloured butterfly" , "Navy bigwig" , "Highest ranking naval officer" .)