Build up a hundred to go on new cure (6)
I believe the answer is:
accrue
'build up' is the definition.
(I've seen this before)
'a hundred to go on new cure' is the wordplay.
'hundred' becomes 'c' (Roman numeral).
'to go on' means one lot of letters go next to another (in a down clue, some letters go on top of others).
'new' is an anagram indicator.
'cure' anagrammed gives 'crue'.
'a'+'c'+'crue'='ACCRUE'
(Other definitions for accrue that I've seen before include "Grow like interest on capital" , "what interest payments do" , "Build up interest" , "Accumulate over time" , "Collect" .)