Build up over time (5)
(Other definitions for amass that I've seen before include "Accumulate over time" , "run up?" , "Sodium chloride" , "Assemble" , "Gathers" .)
(Other definitions for accrue that I've seen before include "Accumulate or grow like interest" , "mount up" , "Fall due; accumulate" , "Collect" , "Grow like interest on capital" .)