So hard entering union circles, ultimately! (4)
I believe the answer is:
thus
'so' is the definition.
('thus' can be a synonym of 'so')
'hard entering union circles ultimately' is the wordplay.
'hard' becomes 'h'.
'entering' means one lot of letters goes inside another.
'union' becomes 'tu' (I've seen this in other clues**).
'ultimately' says to take the final letters.
The last letter of 'circles' is 's'.
'h' put into 'tu' is 'thu'.
'thu'+'s'='THUS'
(Other definitions for thus that I've seen before include "Consequently" , "'So, therefore (4)'" , "In this manner" , "an original Christmas gift" , "Therefore, hence" .)