Two days in A&E with another one tacked on (5)
I believe the answer is:
added
'one tacked on' is the definition.
(I've seen this in another clue)
'two days in ae with another' is the wordplay.
'two' says to double the letters.
'days' becomes 'd' (abbreviation for days).
'in' means one lot of letters goes inside another.
'with' means one lot of letters go next to another.
'another' becomes 'd' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'd' duplicated is 'dd'.
'dd' inserted into 'ae' is 'adde'.
'adde'+'d'='ADDED'
Can you help me to learn more?
(Other definitions for added that I've seen before include "Extra" , "Counted up" , "Said" , "Summed up" , "Appended; totalled" .)