It's put up at the entrance, so top rod needs adjusting (8)
I believe the answer is:
doorpost
'top rod needs adjusting' is the definition.
I can't tell whether this defines the answer.
'it's put up at the entrance' is the wordplay.
'it's put up' becomes 'post' (synonyms).
'at' means one lot of letters go next to another (I've seen this in other clues).
'the entrance' becomes 'door' (door is a kind of entrance).
'post' after 'door' is 'DOORPOST'.
'so' acts as a link.
Can you help me to learn more?
(Other definitions for doorpost that I've seen before include "Upright part of entrance frame" , "Jamb" .)