Joined up under first entry (4)
I believe the answer is:
item
'entry' is the definition.
(I've seen this in another clue)
'joined up under first' is the wordplay.
'joined up' becomes 'tem' (I am not sure about this - if you are sure you should believe this answer much more).
'under' says to put letters next to each other.
'first' becomes 'i' (I in Roman numerals eg Elizabeth I).
'tem' after 'i' is 'ITEM'.
Can you help me to learn more?
(Other definitions for item that I've seen before include "Bit of information" , "Two lovers as one" , "Single thing, poor mite" , "Single article on a list" , "Two people considered together (informal)" .)