Build up a majority (5)
I believe the answer is:
amass
'build up' is the definition.
(I've seen this in another clue)
'a majority' is the wordplay.
'majority' becomes 'mass' (I am not sure about this - if you are sure you should believe this answer much more).
'a'+'mass'='AMASS'
Can you help me to learn more?
(Other definitions for amass that I've seen before include "Accumulate, heap up" , "Gather in great quantity" , "run up?" , "Collect or gather" , "store" .)