Apparently it'll add up to a half century (5)
I believe the answer is:
total
'apparently it'll add up to a half century' is the definition.
I can't tell whether this defines the answer.
'it'll add up to a half century' is the wordplay.
'it'll' becomes 'a' (I can't justify this - if you can you should give a lot more credence to this answer).
'add' becomes 'tot' (synonyms).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'to a half century' becomes 'L' (Roman numeral for 50).
'a'+'tot'='atot'
'atot' written backwards gives 'tota'.
'tota'+'l'='TOTAL'
Can you help me to learn more?
(Other definitions for total that I've seen before include "Completely destroy" , "Whole amount" , "Utter" , "Complete, absolute" , "French oil and gas company" .)