Add up 1 + 100 to get score (5)
I believe the answer is:
music
'score' is the definition.
(I've seen this before)
'add up 1 100' is the wordplay.
'add' becomes 'sum' (synonyms).
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'1' becomes 'I' (Roman numeral).
'100' becomes 'c' (Roman numeral).
'sum' backwards is 'mus'.
'mus'+'i'+'c'='MUSIC'
'to get' acts as a link.
(Other definitions for music that I've seen before include "Melody, harmony" , "Pleasant sound (to one's ears)" , "Sonic art" , "Staff will support this" , "Melodious sounds" .)