Political aide, one in black suit mostly (4)
I believe the answer is:
spad
'political aide one' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'black suit mostly' is the wordplay.
'black suit' becomes 'spade' (I've seen this in another clue).
'mostly' means to remove the last letter (most but not all of the word).
'spade' with its final letter removed is 'SPAD'.
'in' acts as a link.
Can you help me to learn more?
(Another definition for spad that I've seen is " railway incident".)