Aye, tread carelessly with the unknown in the immediate past (9)
I believe the answer is:
yesterday
'immediate past' is the definition.
'yesterday' can be an answer for 'past' (in the past). I am unsure of the 'immediate' bit.
'aye tread carelessly with the unknown in' is the wordplay.
'aye' becomes 'yes' (I've seen this before).
'carelessly with' indicates anagramming the letters.
'the unknown' becomes 'y' (algebra).
'in' means one lot of letters goes inside another.
'tread' anagrammed with 'y' makes 'terday'.
'yes' placed into 'terday' is 'YESTERDAY'.
'the' is the link.
Can you help me to learn more?
(Other definitions for yesterday that I've seen before include "Much-recorded song" , "1965 Beatles hit" , "Song (of the recent past?)" , "24-hours ago" , "Stray-eyed (anag.)" .)